Note that subs don't have internal crossovers. The control that's sometimes (mis)labeled as such is only a low-pass filter which rolls off the sub above the frequency set, but has no effect on attached speakers. They would be forced to attempt to run full-range, without the benefits of bass management.
The question about the Y cable isn't entirely clear, but use of a Y to combine two into one rather than split one into two is inadvisable and can lead to at least distortion. The signal coming down one branch of the Y doesn't know that it's supposed to just go into the sub and also runs up the other branch in the opposite direction.
