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Freezer
Since: Jan, 2001

In a lot of sci-fi and some fantasy, TV shows especially, an ensemble cast visits or otherwise encounters people from an alternate dimension/mirror universe/whatever. They will have similar characters in this other place, usually played by the same actors, may or may not be Evil Twins. The part I'm getting at is that often the alternate dimension version of the ensemble is often paired off differently, romantically speaking (to play with pairings that didn't make it into the main canon, or just for kicks, or as one of many details that is different between the main and alternate settings). Stargate SG-1 used to play with this to pair O'Neill and Carter, f'rinstance.
If this somehow doesn't already exist as a named trope (I didn't look, I have no idea what to search for), I propose: "Love Counterfactually".