ok, i was looking some stuff up, but its from the internet, so, grain of salt & all...
on amps w/more than one input (4 input fenders, marshalls, etc, 2 input fenders & marshalls, etc...), but WITHOUT reverb, plugging the guitar in input 1, sending a jumper cable from input 2 into input 1 of the 2nd channel (like the vibrato channel on a fender) will chain both sections together. this gives you the ability to use all the tone controls interactively, as well as adding half a tube into the mix for an extra gain stage. if we did the same thing w/2 amps instead of one amp w/2 channels (not a channel switcher, just two separate channels), its called 'daisy chaining', i can only assume (dangerous word there) that the inputs are wired in parallel & that's why this works, but it does work, & can give yer amp a little push, too. i do this w/my '60 ampeg & my '68 bassman, if i'm not running them in stereo, that is...
oh, yeah, doing this on a two channel amp w/reverb will put yer signal out of phase, thinning out the tone. i don't know if the same happens when one amp has reverb & the other doesn't, i don't own any amps w/reverb. also, ground loop hum is (sometimes) a consequence, in which case, well, there are all sorts of remedies, some safe, some not...use common sense is all, ya know?
um, sorry, clarification needed: by "OUTPUT" you don't mean SPEAKER OUTPUT, right? you don't want to plug the speaker out of any amp into the front end of another, its a good way to blow up both, from what i've heard/read...the input daisy chain avoids this (again, i assume) because of the parallel input thing...