I so sympathize with where you're coming from, Ellen!
We often travel with another couple and usually get one bill. When we do, we each calculate what our actual share is of the food, but split the wine bottles equally. We also split the total tip equally (easier to calculate that way). That works really well for the four of us.
A couple of years ago, we traveled with a different couple, assuming that we would be handling meal bills the same way. The husband would grab the bill and immediately calculate the bill in half, for each couple to pay. It didn't work for us - the bill was always skewed by several pre-dinner drinks and driven upwards by wine more expensive than what we'd usually order. It was embarrassing for me to bring it up, but after a couple of days, I started asking for split bills at the beginning of the meal, at the time of ordering. I also quickly got into the habit of selecting and ordering the first bottle of wine and putting it on our bill. We would not pick up the second bottle. The compromise worked well and we didn't pay as much as at the start. This was in France and the requests for separate bills were accommodated with no issue. In the rare instances where there was a "couvert", it was added in appropriately.
Since you're already aware of the issue, let the other couple know how you would like to approach bill paying during your trip. You should have this discussion before you go. You could use the unfamiliarity of paying for meals in Italy as your reason to open this discussion... ;-)
Make sure that you let the waiter know at the time of ordering how you want the bill done. The only thing I'd watch in Italy is that if a "coperto" (cover) is added, that it be added per head on the appropriate bills. I'd also watch how the "servizio" (tip) is charged, if it's added to the bills.