The 22-2 does not have the ability to overdub to an already recorded track. For example, you can't record piano and then overdub your vocal using that same machine. You'd either have to do it "live", both parts at the same time or ........ get 2 TASCAM 22-2, record piano on the 1st one and then dub that onto the second machine's track, while doing a vocal to that same 2nd machine's other track. You'd then, for instance, have piano on on the right and vocal on the left, both which can be mixed later. Actually, you can even track both to the one track and leave the other for other parts. Quite a bit of possibilities there, with 2 decks. Bouncing like that once or twice will not degrade the signal significantly. Tape hiss will not really be an issue if you know what you're doing. Personally I would not use a dbx NR system with such a setup. Try it first without and see how you like it.
As far as the TASCAM 32-2 is concerned, I'm fairly certain that it has the overdub capability (simul-sync) and in which case you don't need a 2nd deck. Can anyone please confirm this ?