Have flown transatlantic in separate cabins on AA a few times. My wife was in first class on award ticket, while I was in business upgraded with miles from paid economy. She has a back problem which makes sitting for long stretches uncomfortable, and American only has flat seats in first class. This was before AA added a $500 "co-payment" for international upgrades and the upgrade was better value than a free ticket. Probably would not make the same decision today.

One drawback to doing this on a US based carrier is that passengers are not permitted to move between cabins during flight, and we were not granted any exception, although sometimes a flight attendant would ferry a message back to me.

dennis