My wife is insistent that we sit together. I'm sure she would make it apart, but we haven't wound up there, though it has been close.
We always select seats together so it's rarely a problem. Usually it's if we get upgraded to FC and there are no two seats together. Since her profile is window and mine is aisle, this gives us more flexibility to change.
I would never poach a seat, but one of us nicely asks if someone will change. We've gotten no and that's okay, but somebody has always accommodated us. We don't accept an upgrade if there is only one seat. We've had GA call us up and ask, but I always check the board, if there is one, to see if this situation might occur and I have gone up and told the GA we would not take one seat.
One time the next person on the upgrade list was standing there and we told her it was her lucky day. She was thankful and had drinks sent back to us.