frame 1 goes to GPU1,
frame 2 to GPU 2
what ever faster will stop and wait until the slower GPU finishes,
then frame 3 goes to GPU 1,
frame 4 goes to GPU 2,
on and on
a GPU on a slower connection would need testing to see if there's any real world advantage on a case by case basis
if you own the hardware already there's no harm in hooking it up and testing...