I have 2 guitars that I switch between. Is there an adapter I can plug into the back of the Harmony-G(XT), VoiceLive2 or VoiceLive Touch that will allow me to switch between the two guitars without having to unplug / replug all the time?
Yes, you'll need to use a guitar switcher; also sometimes referred to as an A-B box or A-B-Y switcher. , which is a type of accessory product manufactured by several manufacturers (though not TC).
Most guitar dealers should carry one, and would be able to advise you further.