Is it OK to charge 2 plugins from the same GFCI?

There's no issue with regard to sharing outlets on the same line; however, you may not have the capacity.

12 amps is the regular draw setting. That works fine on an ordinary 15-amp line (since it allows for the necessary 20% safety buffer).

To support 2 Prime at the same time, you need a 20-amp line. That requires reducing draw to the 8-amp setting for both vehicles. No big deal, it just extends the time... which is fine for overnight charging. Not all garages have 20-amp capacity though. Check the circuit to be sure.

btw, only a single GFCI is needed on a line to provide protection operation.

 

If you find that your garage is served by multiple breakers, you can add a GFCi outlet for $20 in parts. If you only have one breaker I suggest one car at a time.

 

It won't damage anything, if that's what you mean. But it will trip the circuit breaker, since they will draw more combined amps than the breaker (or the wires) will allow.

 

And to clarify that a bit:
If wired properly, one GFCI can protect other outlets on the same branch circuit too.
In most cases, ALL the outlets in the garage are on the same branch circuit and they are all protected by one GFCI device, either in the breaker itself or in one of the outlets.

If yours is wired that way, only one circuit for all of the outlets in the garage, then you would have to plug in your two vehicles one at a time or revise the wiring with a new circuit.