The way I have it noted (maybe right, maybe wrong):
Say you have two second chances in play, we'll call them SC-A and SC-B
Something happens, and more than one of your units leaves play at the same time
SC-A and SC-B trigger, you can order them however you want, lets order SC-A to resolve before SC-B does
SC-A resolves. It then randomly chooses amongst the 'units that left play' and returns a unit to play (as long as its still in the zone that it went to after leaving play).
SC-B resolves. It then randomly chooses amongst the 'units that left play' and returns a unit to play (as long as its still in the zone that it went to after leaving play). Note: It could 'randomly choose the same unit chosen by SC-A, at which point it would 'have no effect' when it resolved.
Rules about SC's:
It doesn't matter who controls the SCs, or whose turn it is when they trigger, no player ever gets to make a decision about what is returned.
If the ability 'has no effect' then that SC will continue trying to return something to play, as its once-per-turn has not yet been used up (it tried to use it, but couldn't, so hasn't yet been used 'once-per-turn' yet).
Based on other cards, its possible for SC-A and SC-B to both 'fail to return something to play' in certain situations, and for them both to continue trying to return things later in the turn.
Every SC you control triggers each time any unit(s) you control leave play, whether it has already returned something or not. When it resolves it decides if it should return something to play or not.