I don't know how to address two invites to FI Uncles, they are both older men too.
Case 1: Uncle has been with his girlfriend for 10+yrs they just moved in with each other. I think this one goes as one invite to their address as Mr. Joe Doe and Ms. Jane Smith, am I correct?
Case 2: Uncle started dating a very close family friend about 4 yrs ago. She would have been invited on her own even if not dating the uncle. She has two kids, and they are invited, but they don't live together. Her kids are younger and they've decided too much change too fast wouldn't be good for them. We don't want to insult their relationship by simply sending +1 invites to their seperate addresses because we clearly know they are together. Soooooooooo here is my guess on how this goes, one invite to the Uncles address as:
Mr. John Doe and The family of Mrs. Sarah Smith