Not sure which Bowers sub you have, but I would assume the inputs get summed to form a single LFE signal. If one of them has no signal, it will just use the one signal present. You shouldn't need to use a Y connector or similar setup to create a second signal, that would essentially do the same thing. You can try it, but I doubt you'll gain any volume.
Your AVR's bass management should have a high pass filter option somewhere for your mains after you set the crossover frequency. I agree with the previous poster, you will struggle getting it to sound right at different volumes if you feed full range to your mains.