I'm sure that amp sums its inputs. You can run a single RCA into the left input and it'd be the same difference as splitting (halving) the signal into both inputs and letting the amp re-combine them. It's mono out regardless. I doubt you could tell the difference in a blind test.
Having said that, if it were my sub, I'd probably still use a Y cable
like this one just to satisfy my sense of symmetry. I'm a little OCD like that though. Realistically there's no benefit, but by golly I like using things that were made to go together.