I made a mistake -- for some reason I assumed this was a tube amp. (I don't
know why.)

Regardless, if the output levels are closely matched, one amp /will not/
pump current into the other. But to gain any increase in power, you'd have
to use a lower-impedance speaker.

There is also the possibility that, even with the levels matched, the amps
might not "like" looking at each other.

"Buy a decent amplifier that delivers the power you need." -- The Lady from
Philadelphia