parallel lives

Parallel lives are the simultaneous but separate existences of two or more people whose paths run alongside each other without intersecting, or whose experiences mirror each other despite never meeting.

The phrase often suggests lives that could have converged but didn't: They lived parallel lives in the same city for years before finally meeting.

It can also describe how the same person might have turned out under different circumstances—the roads not taken: She sometimes imagined the parallel life she'd be living if she'd taken that job in Paris.