Using a second amp as alternative to loud speaker

Should work OK, especially if the second amp has a line-level input. Mostly a matter of getting the gain right so there's no distortion and, depending on where you put the amps, the volume balance between them so from out front you sound like you're coming from where you are, not somewhere off to one side if you see what I mean.

A couple of powered PA speakers, stands and a small mixer is probably the better way to organise things, especially if you find you need to add some foldback later on, but using two amps in the way you suggest isn't that different to a stereo PA really. It's just using one of the amplifiers as a pre-amp as well as a power amp, rather than a mixer being the pre-amp and the speakers only having power amps.

Watch out for earth loops between the amplifiers though. When using multiple amplifiers/powered speakers star earthing everything back to one mains socket if possible is the way to go so long as you don't need more current than the socket provides. Connecting a DI box with a switchable earth float to the line out of amp A then the XLR lead from the DI into the mic input of amp B is another solution to earth loops that generally works.