Vonnegut had a word for that in one of his books, for lovers who died within a month of each other. I just flipped through my copy of Cat's Cradle, where I swear it's from, but I couldn't find the passage marked. Lemme google.
After a little googling: the word you're think of is "Granfalloon" which means ''a false karass, [...] a seeming team that [is] meaningless in terms of the ways God gets things done"
So, it looks like we're both getting the word from the same book, if only one of us could find the right word.
Edit the second:
"A
duprass is "a karass composed of only two persons". The members of a duprass die within a week of each other. [ 41 ] A duprass "is a valuable instrument for gaining and developing, in the privacy of an interminable love affair, insights that are queer but true." It "is also a sweetly conceited establishment." [ 55 ] "
*phew* I knew it was Cat's Cradle