I think the most important thing is to discuss this with your friends ahead of time so everyone knows how it will be handled. Otherwise, you could end up not being friends anymore! Some people don't mind just splitting the tab, while others won't want to do that, especially if someone drinks a lot more wine or always orders the most expensive thing on the menu.

One thing to remember is that in Italy, the tax and service charge is already in the amount shown on the menu. Therefore, it's really pretty easy to add up what each couple owes. If you want to use a credit card without getting separate checks, just keep a list of how much the other couple owes you. I wouldn't think it would hurt to ask for separate checks, the worst they can do is refuse.

Most important is to talk about it with the other family before the trip.

Easy enough to keep a small notebook and jot down for each meal approx. total for each family, and who paid the bill that meal. Tally at the end of the trip and low guy pay difference to high guy. Doesn't matter who is paying with what method, and doesn't bother the waiter with extra work.