Tell the 'charger' to run at 20A
You do not need 30A of charging. You can git-r-dun on a 20A/240V circuit. Really.
Your "car charger" is actually an EVSE and not a charger. Here is how those really work.
The EVSE tells the car how much power it is allowed to draw. Simply tell the EVSE it's on a 20A breaker instead of a 30A breaker. Since EVSEs by law run at 80% of breaker, that will derate it to 16A (3840W) instead of 24A (5760W). If for some bizarre reason this EVSE does not allow you to configure that, get a better one. They all use the same connector (J1772).
"That was easy"
You have several options. Paralleling is not one.
Also, I hope you're not married to that charger, because it sucks. It requires a neutral wire, and there's no earthly reason for an EV charger to need neutral. Get another one, or push back on the builder and ask "How do I wire this 240V-only?" They're crazy not to - all the market growth is in Europe, Australia, Asia or the middle east. Tell them you're taking the car to Europe.
While you're talking to them, ask them if they support 380V (middle-east 2-leg) or 480V (industrial) because that's a backup plan.
Also ask them if the charger can be programmed to only draw 20A, because that solves it right there.
Just run the darn cable
No electrician would install your ideas, so you're clearly doing this yourself, armed with "a little knowledge". And you're no pauper if you're getting an electric car. The proper 10/3 cable is just not that expensive when you DIY, so stop being cheap and go do it.
In fact, with modern pricing, you can get a 2-2-2-4 Aluminum feeder cable for the same price as 10/3, and that is legal for 90A. That means you are ready for "next-generation" multi-EV charging at 80A shared.
Now you're playing with power
Your power draw is 7200 VA (240V x 30A). Instead of carrying it that way, use two common 10KVA transformers to step it up to 480V x 15A for transmission. That's within the range of your 12/2 cable. If the charger can intake 480V directly, you'll only need one transformer. Ground remains ground. You will need a grounding rod out at the vehicle.
If the charger really needs neutral, then your second 10KVA transformer out at the charger can be jumpered for 240+120V output, giving neutral.
Now you're playing with funny fractions of power
If the transformer can take a Euro voltage of 360-417V, you do something vaguely similar by using transformers in a buck-boost fashion to step up a 240V/20A to higher voltage (still at 20A). The direct supply needs to be breakered at 20A, and since the connection is series, they will suffice to protect the wires. The boost transformer(s) will need their own appropriate breakers.
Everything from here will probably be nixed by your electrical inspector, and/or your insurance company will decline to pay for the fire, leaving your wages garnished for a long time (can't escape an illegal act with bankruptcy).
Oh, you're going to do it anyway?
That's foolish, but at least you can shave some excessive and gratuitous foolishness by complying with the rules for paralleling (only violating the rule that says paralleling can only be done on #0 and larger cables, and with equipment approved for paralleling). First, you spread conductors, not gang them: each cable gets one of each. Phase L1 goes on black of both cables. Phase L2 goes on re-marked white of both cables. This balances currents in each cable, and reduces vibration and eddy current heating (which will be serious at this amperage). Grounds are grounds. Neutral would not be used, which is why to push back on the charger manufacturer and tell them you want a no-neutral charger.
Further, every conductor must be fused individually including neutral, no matter how hard that is (except ground). Your breakers provide hot protection, if you use neutrals you can kit-box something to protect them, or you can add a small three-phase subpanel and use one phase for neutral. Remember, this is still super stupid.
Intentionally overload one 12-2 cable
This is also bad, but not as bad as paralleling, but there may be a way to "make it legal". If you look in the code book, 12 AWG wire is legal to 30 amps if you can run it at 90 degrees C, but this is forbidden in 240.4(D). So we can make this legal-safe except for the 240.4 violation. The first impediment is terminations (breakers and plugs). You could solve at least that problem by pigtailing with 10AWG wire, and use a 90C rated splice to transition to 10 AWG pigtails.