Short story: aliens that merge into one

[iodama]

I think I read this short story before 1989 in Asimov's SF Magazine (which started publication in 1971 and I was reading back through old ones in a collection, so it could be any time in the 70s or 80s). It concerned a young adult male protagonist (on Earth, who thinks he's human) who meets an "uncle" who looks a lot like him, from whom he learns that he is not human, but an alien passing among humans, who goes through a maturational process whereby two genetically related individuals fuse into one, and his "uncle" who is looking younger by the day and he are fated to merge into one person.

( Key scene description )

Anybody recognize this story?