Hi again
It is not a proper way to connect the out put of some difference amplified signals together . hence you should use a mixer . a mixer in input before amplifying the signals . thus you'll need just an amplifier . not two amp .
There are many ways available for this aim , such as a summing amplifier .
Anywhere if you decided to do that ( connecting the out puts together ) i don't think that if your key has enough surface there would be any problem . ( it will be negligible )
Best Lucks
Goldsmith