If you have a factor pedal at the end of your chain, most algorithms I can think of in the pedals should sum your two inputs to one output, as long as output_1 is your output. The pedals know what is plugged in and change the algorithms' structure accordingly. The only exception I can think of off-hand is Octaver in PF, which is always dual mono.
Of course, this is moot, if you don't have a factor at the end of your chain. It might be worh trying out though.
-Russ
