To be clear, you want to use Y-adapters to combine the stereo pre-outs into a single long RCA to the sub, and then "uncombine" them with another Y-adapter to send signals into the left and right inputs on the sub?
First off, I'm not sure why you would need to uncombine back into two inputs at the sub. Just leave them as a single cable and use whichever input you want on the sub. Second, a splitter won't be able to create a new stereo image from the "combined" input signal, and a single sub won't care about having stereo inputs or having half the signal on each of the L and R inputs.
On top of that, it's unlikely that a simple splitter will effectively unify the stereo output signal from the Luxman. At the very least, I think you'd need a certain amount of in-line resistance on each of the two output channels before combining them, as without that you could create a short back to ground on the other channel. I doubt this would damage anything given the low level of a pre-out signal, but you certainly wouldn't be getting the whole signal over to the sub, and bass is the most likely frequency range to be hurt by an inadvertent short or ground loop.
Note that going the other direction (using a Y-splitter to split a single output to drive two subs) works just fine.