Lots of concerns. Outputs of two amps (at least solid state amps) should never be connected together, and what you are proposing to do does exactly that.
It's hard to predict what will happen without specific design details on each amp, and what will happen will depend on whether the one that is not being used is on or off, but it is highly conceivable that damage to either or both amps can result.
Even if damage does not result, if the unused amp is on, it would have a very low output impedance, likely a small fraction of an ohm if it is solid state, which would cause excessive loading of the amp that is in use. If the unused amp is off, its output impedance is probably undefinable, with unpredictable loading effects on the other amp.
Don't do it!