sgarecfinders | Parallel universe, character missing (Reply)

One of the requests below resulted in this rec:

"You Might by deltacephi: After a run-in with the time jumping Daedalus, Rodney goes to Earth to look for John Sheppard, a fellow with the ATA gene. http://www.deltacephei.com/sga_youmight.html"

This has got me craving something very specific: stories with visits to/from parallel universes where a character meets someone who doesn't exist in their world. Even better if they then try to locate that person in their universe.

I'm not looking for original characters - I'm looking for something like visitors from a parallel universe arriving in Atlantis, asking for Sumner and not knowing who Shepard is because their Shepard was killed in Afghanistan. Or a Rodney visiting our universe, meeting Carter, then going back to his universe and trying to find her.

[I checked the archives and there was a similar request in February for Rodney stories, but I'm interested in any character]