Good news: there is an intersystem bonding terminal that'll fit on your existing GEC
The good news for your situation is that Arlington makes a bonding device, the GBB5250, that will fit on a 2/0 grounding electrode conductor. You'll have to cut into the conduit housing the GEC to install it, so that you can gain access to the wire inside. However, the provided cover does have conduit entry/exit points, though, so it should be possible to install it without having to sever or disconnect the GEC, or install a replacement length of conduit for that matter.
Bad news: your service configuration is no longer permitted as of the 2020 NEC
The bad news for your situation is that your overall service equipment configuration, with a Class 320 meter base providing 4-wire, 200A feeders to two 200A main breaker panels, does not comply with the new 2020 NEC requirement to provide the dwelling unit with a single emergency disconnect that's operable from the exterior of the house so that firefighters don't have to risk pulling a meter under load. Fortunately, that's fixable in your situation without rerunning any of the wiring.
Ugly news: your current configuration doesn't even comply with the 2017 NEC (or earlier editions, for that matter)
However, your electrician found a way to screw this up even under current Code. As it turns out, the green wires from the meter base to the panels are undersized! This is because the conductors from the meter to the service disconnect are service-entrance conductors, not feeders, and as a result, the normal EGC sizing rules don't apply to those green ground wires. Instead, they are a form of main bonding jumper, and thus are required by NEC 250.28(D)(1) to be sized according to NEC Table 250.102. In your case, this means that the electrician should have used 2AWG copper wires for the bonding jumpers, as the ungrounded service-entrance conductors for each service entrance are 4/0 copper.
As a result of this, bringing this setup to Code is going to require a bit more work than just slapping an IBT device in there and calling it a day. Since your existing service-entrance conductors (meter base to inside panels) are 4/0 copper, we can take one of three paths to fix this, since Green Mountain Power requires a lever bypass on Class 320 meters yet is fine with main bonding points in meter bases, as per their service handbook:
Keep all the existing hardware, but replace the 4AWG copper bonding jumpers with 2AWG bonding jumpers to make them meet the NEC bonding jumper sizing requirements. This is the cheapest/easiest route, but doesn't provide the benefit of a single disconnecting point for all power.
Keep the existing conductors and fit a single-400A-disconnect meter-main that supports a pair of 200A branch breakers, treating the existing conductors as 200A feeders. Thankfully, even with the existing oversized hot conductors, the 4AWG green wires are large enough to be EGCs, so this approach requires no wiring changes, but it does have the drawback that it's severely limiting in your choices for a meter-main.
Fit a single-400A-disconnect meter main and use the existing conductors as feeder tap conductors. This provides a couple of additional meter-main options, at the cost of having to replace the existing 4AWG copper bonding jumpers with 3AWG wires (as they now need to be adequately sized for a 400A feeder). It also invokes the 10' feeder tap rule in NEC 250.24(B)(1), which is legal in this case, but has a habit of causing heartburn to home inspectors and other such residential-only folks who are unfamiliar with the feeder tap rules.
If you don't mind the current configuration...
If you have some reason to want to prefer relying on the "rule of six", you can still do that for now, although it no longer will be an option once your AHJ adopts the 2020 NEC, so your time window for fixing this is limited. The good news is that simply replacing the 4AWG bonding jumpers with 2AWG jumpers (i.e. correctly sized for the oversized service-entrance conductors) will do the trick for now.
Taking the conventional approach
While using a 400A single-disconnect meter-main with two 200A branch breakers is the most conventional solution to this issue, and requires work in only one place (replacing the meter base), it poses one major issue: equipment availability. In particular, there are only two ringless/lever-bypass meter-main combinations that can do this, namely the Siemens MC2442B1400SDL and the Square-D QUM400CL. The former is the less expensive solution of the two, uses a pair of Siemens QN2200R branch breakers, and also provides 16 spare spaces on the outside of your house, but does not series rate with the QOM2/QO breakers in your existing panels, which may be problematic, depending on the fault current available from your utility service.
The latter is costlier to begin with, requires the field fitment of a BMK2Q400 kit in addition to a pair of QBL22200 or QDL22200 (depending on interrupting rating requirements) branch breakers, and does not provide any extra breaker spaces. However, it is possible to get a full series rating on the all-Square-D solution, which allows it to handle fault current levels far in excess of what Green Mountain Power can provide. It also doesn't have the right-sized lugs for all the load wires it needs to handle, so you'll need to use a mechanical splice connector or split bolt that can handle 3 4AWG wires in order to "pigtail" the two EGCs into a single grounding conductor that can then go into the other small grounding terminal as one of the two provided will be occupied by the GEC, which is too small for the large lugs on the QUM400CL.
Tap-dancing our way around availability issues
If you cannot obtain either of the above pieces of hardware, we'll have to do some more work here. First, the 4AWG grounds in the service-entrance wiring will have to be upsized to 2AWG grounds, so that the 10' feeder tap rule from NEC 250.24(B)(1) can be applied to your situation:
(B) Feeder Taps. Conductors shall be permitted to be tapped,
without overcurrent protection at the tap, to a feeder as specified in 240.21(B)(1) through (B)(5). The provisions of
240.4(B) shall not be permitted for tap conductors.
(1) Taps Not over 3 m (10 ft) Long. If the length of the tap
conductors does not exceed 3 m (10 ft) and the tap conductors
comply with all of the following:
(1) The ampacity of the tap conductors is
a. Not less than the combined calculated loads on the
circuits supplied by the tap conductors, and
b. Not less than the rating of the equipment containing
an overcurrent device(s) supplied by the tap conductors or not less than the rating of the overcurrent
protective device at the termination of the tap
Exception to b: Where listed equipment, such as a surge protective
device(s) [SPD(s)], is provided with specific instructions on mini‐
mum conductor sizing, the ampacity of the tap conductors supply‐
ing that equipment shall be permitted to be determined based on the
(2) The tap conductors do not extend beyond the switchboard, switchgear, panelboard, disconnecting means, or
control devices they supply.
(3) Except at the point of connection to the feeder, the tap
conductors are enclosed in a raceway, which extends from
the tap to the enclosure of an enclosed switchboard,
switchgear, a panelboard, or control devices, or to the
back of an open switchboard.
(4) For field installations, if the tap conductors leave the
enclosure or vault in which the tap is made, the ampacity
of the tap conductors is not less than one-tenth of the
rating of the overcurrent device protecting the feeder
Informational Note: For overcurrent protection requirements
for panelboards, see 408.36.
Otherwise, the hots would be insufficiently sized as a 4/0 Al wire with 75°C terminations can only carry 180A, and 240.4(D)'s "round up" rule cannot be applied to feeder taps. Furthermore, 4AWG grounds are one size too small for a 400A circuit -- table 250.122 requires 3AWG grounds on a 400A circuit.
Once the wiring is redone, then the meter socket can be replaced with any ringless, Class 320, lever-bypass meter-main that has a single 400A service disconnect in it. This includes the Square-D QUM400CL from before, the Siemens MM0202B1400SCL or SDL, or the Milbank U6227-X-400-K3L. If you use the QUM400CL, though, you'll still need that splice and pigtail for the grounding wires, although it will need to be able to handle 3 3AWG wires in this case.
Making all the right connections
You will need to adjust the locations of the various conduits on the outside to accommodate the new meter-main if you go down that path. It should be possible to fit it in the space available; however, you'll need to either have the utility move their service to the left, or adjust the GEC conduit route to the right and have the feeder conductors run directly into a panel instead of into the T-body, with the other feeder simply passing through the panel as permitted by NEC 312.8's rules on feedthrough conductors. (You'll also need to blank off the unused T-body port if you do this.)
One more thing...
While you have the right idea with the tape (in fact, NEC 408.3(A)(2) requires insulating barriers on all ungrounded service terminals in panelboards), there is a better solution to that problem. Due to the aforementioned NEC (and UL) requirement, Square-D makes retrofit service barrier kits for their loadcenters; simply order a pair of PKSB1HA kits from your local supply house and fit them in place of the tape using the supplied instructions, with the service turned off by the utility of course!