JohnK is right.
I've used a y-splitter for my sub just because the cable i bought came with one.
The other reason you might use a single input, as opposed to both on the back of a sub, is if you have 2 subwoofers and wanted to distinguish them as right or left out of the processor/receiver. Of course this assumes the media you are playing has a distinguished track for 2 (right and left) subs to make any difference but so far, i haven't heard of any.
Considering the general inability to localize low frequency sounds at closer ranges, i don't even think it would matter.
