Welcome, and just so you know this site is geared toward car-computer integration and you may get better responses from a car audio site (links are in the Car Audio stickies section). As far as what you're trying to accomplish, I guess the obvious question would be why? I suppose it's easily done as long as you're using outboard amps. Just use a rca y adapter for the 2 outputs of each deck to the 1 input of the amp. I don't think you'd need a switch, just keep them both on. If you get some noise, just turn off the one you're not using.
I still have to ask, why would you want to do this?