Two Sources to One Powered Subwoofer

I would like to know how to hook up two sources to one subwoofer. Right now I have a Sony STR-GX10ES two channel receiver with two rca outputs connected to my Quad Lite powered subwoofer. I just hooked my old Denon AVD-2000 Dolby Digital Surround Decorder and a Luxman F-114 three channel amp (to power the center and surround speakers). I then connected the left and right channel outs on the the Denon processor to the inputs intended for a graphic equallizer on the Sony two channel receiver. I have my source equipment (DVD player, Directv receiver and CD player connected to both the receiver and the Denon processor. When I want to listen to Dolby Digital, I use the adaptor switch on my Sony receiver and turn on the Denon DD processor and the Luxman and voila, I have 5 channel, Dolby Digital. That’s right, I said 5 channel, not 5.1 which is why I would like to connect my subwoofer to the Denon processor as well when I watch a movie or some Directv. But most of the time, I use the Sony two channel receiver when I listen to cds and some tv which is why I want the sub connected to the Sony as well.

Should I simply use two rca y-adaptor cables to connect to my subwoofer OR should I use an rca A/B switch, OR something else that I am not aware of?
You should not connect two sources to a single input. Get a switch.
Radio Shack has a stereo audio selector that I use to switch my subwoofer between my HT and two channel.
I don't think your Quad has a build in Xover, so when you play two channel your sub would work with the full range signal, unless your Sony has a 4 outputs and a build in crossover. This is far from ideal, as the sub would overlap with the mains in the 60-120 frequency range. To do this right you need an external crossover.

The NHT X2 is an cheap active crossover that would meet all your needs - it has 2 channel and LFE inputs, allowing you to integrate stereo and surround sources with a single (or dual sub). Unfortunately they are no longer manufactured (I just sold mine).

Your other option is any other crossover and a switch.