I don't think that's a good idea, and here is why:
Outputs are low impedance and must only be connected to high impedance inputs, so never tie two outputs directly together. If you do, then each output tries to drive the very low impedance of the other, forcing both outputs into current-limit and possible damage. As a minimum, severe signal loss results.
So in short, never use a Y-cable to combine signals; they are suppose to be used to split the signal. For example, using it at the subwoofer input on the receiver would be okay in order to split the signal into mono and utilize dual subs.
Here's a good read on the subject:
Why Not Wye?