I think this might actually be covered in the ED FAQ. Whatever the case, I have read of people keeping one plate, but I think you have to specifically request it (I don't know about getting both though). IIRC they are going to change the license plate frame in the front to the US-style frame, so you would have to order that and change it back.
In Calif. it's not legal to run the European front plate like that and I doubt they would let you get away with it for long. I have seen lots of crazy stuff on cars here, but never that.
I'm sure someone that has actually done the ED will reply soon enough.
Anyways, good luck with your ED. I didn't have the time to do it that way this time, but I am planning to do ED next time... what a great way to buy a car!
Good luck
--SONET