We'll be traveling with another couple in Paris. When we go to restaurants at home, we typically put two credit cards on the bill and waiters (and waitresses) know that they should split the bill between the two.
Is this done in Paris or would we be committing a faux pas?
Totally fine to do that.
Since in just about every place I've been (if not all) the wait staff uses a hand held device to process credit cards, this might be a bit difficult. Alternately, IMO it shouldn't be a problem to politely request separate checks for each couple, but do it when you first order. The nice thing about the handheld devises is that your card never leaves your sight.
When they bring the hand-held machine for processing the credit cards, just tell them how much you want to charge for each card. They'll just punch it in and process it for each card separately. If you want to leave a tip, use coins/cash. I did this all the time while living in Paris.
We have done this many times with no problem when dining out with other couples. It is especially easy if just splitting the bills 50/50. They just run each card for half the total.
This is an easy and accepted practice.