SCE may not let you cheap out on this
While NEC 230.40 Ex 3 would permit you to hang a socket at the garage and have separate service entrance lines from that socket to the garage and the house, that may not fly under the SCE ESR-5 section 3.0 and 3.1 rules on meter switches:
3.0 Meter Switch
For each and every meter, the contractor shall furnish and install a switch, or other approved
disconnecting means with overcurrent protection. This is referred to as the “Meter Switch." It shall
be installed at the same location and directly adjacent to the metering and service equipment. The
meter switch shall control all of, and only, the energy registered by that meter. Where permitted by
local code or ordinances, the meter switch may consist of a group of switches or breakers per
National Electric Code (NEC) 230-71. A separate meter switch will be required for each separate
service of any group where the loads are totalized by a single meter.
3.1 Number of Switches
Every meter or service shall be furnished with a meter switch as described in Section 3.0.
The NEC allows up to a maximum of six switches or disconnects to constitute the main
overcurrent device for a single commercial or industrial service. A residential service may
have a maximum of two switches or disconnects for each single-family service.
As a result of that, we'll have to start with something SCE will definitively let you install, even if it is a vastly more expensive setup than what you were originally contemplating. We're also going to assume from your initial choice of meter socket that this is an underground service dip from the pole; if it's an overhead service instead, then you will need a unit that accepts an overhead conversion gutter, such as an Eaton HP40 or HP402442, or a Square-D CUM400CB.
The first component we'll need is a 400A, EUSERC-compliant, single disconnect meter main. This is a very expensive piece of equipment (as in several thousand $), but it is quite important in that it's the only way we can provide 300A to the house while meeting SCE's rules about meter switches and the NEC restrictions on multiple feeders to a single structure.
Once we have that, our path then is determined by whether the meter-main you selected has a loadcenter interior in it or not. If it does, then we can simply get two 100A breakers for it (or a 125A and a 100A, if your garage feeder turns out to be bigger than 100A) along with a matching pair of 225A subfeed blocks to provide an effective 400A tap from the panel busses to the house. In this configuration, the garage panel is a 200A main lug (or main breaker converted to main lug) unit, configured as a subpanel, with a 100-125A feed from the main panel provided to it, and the solar breaker lives directly in the main panel.
If it doesn't, then we'll need either a three-pole power distribution block (UL1953 listed) with 2 250kcmil line and at least 3 250kcmil load ports or a 400A 120/240V tap (terminal) box and a conduit nipple + fittings, along with more 250kcmil Al to jumper between the load lugs of the main breaker and the tap box or PDB, to tap the feeder to provide power to the garage. Since we are invoking the 10' feeder tap rule in NEC 240.21(B)(1) for the garage feeder in this case, we must run said feeder in conduit (using 250kcmil Al conductors and a 4AWG Al ground) instead of using a SE cable, and position the garage panel so that it is close enough to the main panel for said 10' rule to apply. Furthermore, we must use a 200A main breaker panel for the garage panel here, also due to the tap rule coming into play. This additionally means that the solar breaker must live in the garage panel.
Either way, from the garage to the house, we'll need to lay a 3" PVC conduit with 6 250kcmil wires (a pair for each hot and a pair for the neutral) and a 1AWG Al ground in it. At the house, we'll need a 400A, 120/240V, NEMA 3R tap/terminal box to combine the parallel conductors along with 2 200A main breaker panels, configured as subpanels. Furthermore, since we're again invoking the aforementioned 10' feeder tap rule for the connection from the tap box to the house subpanels, we again must run the tap connections to the breaker panels in conduit, and position the house panels accordingly.
Note that neither of these setups requires an emergency disconnect at the house since it's being fed from service equipment located elsewhere on the property. Grounding is not hard, either; the garage's ground electrode is wired to the main panel in this configuration, while the house's grounding electrode has a conductor run from it to the Intersystem Bonding Termination device at the house, which then has 4AWG tap conductors run from it to each of the two house panels.
Don't get gassed!
The LA Basin is an oil and gas producing area (still!), and one of the drawbacks of that is that you need to be attentive to the possibility that methane gas may try to seep into your electrical system via underground conduit runs, with obviously bad results. The normal solution is to use extended range explosionproof Y seals (Eaton/Crouse-Hinds EYSX) in between your terminal adapter/expansion joint threads and the panels or tap boxes themselves, so that these metal fittings get grounded correctly. Note that they need to be filled with the appropriate sealing compound to do their job!
Don't let your electrical system give you the loose lugnut, either!
If you're planning to install this yourself, note that you'll need an inch-pound torque wrench to meet the NEC 110.14(D) terminal torque requirements. This is a good idea even if your jurisdiction doesn't enforce this rule, since you really don't want your electrical system to do to you what Greg Biffle's loose lugnuts did to him!
If SCE wasn't so picky...
If SCE lets you apply NEC 230.40 Exception 3, then things become simpler. The meter box gets replaced with a Milbank U5056-O fitted with a K5049 triple tap kit, and the various 200A panels are fitted out as main panels instead of subpanels; also, there is no need for equipment grounding conductor runs from the meter to the panels, as they are all main panels now. However, this does introduce the need for an emergency disconnect at the house instead of a simple tap box. While one could use a fused disconnect for this, given the cost of dummy fuses these days, it's cheaper to use a non-fusible disconnect switch. That said, 400A NEMA 3R non-fusible disconnects are not cheap or easy to find any more -- the Square-D HU265R is the only one that I can get a reasonable in-stock price for, and that requires you to fit a H600SN neutral kit to it for your application.
