Is there a reason you've chosen to put the two mic positions in separate samples instead of combining them in a single sampler - http://forum.hise.audio/topic/75/multimic-samples
If you combine them my general purpose mix mixer script will be a good starting point - http://forum.hise.audio/topic/312/general-purpose-microphone-mixer
If you still want to keep them separate then you just need to link a knob to the gain parameter of each sampler. Or you could add a constant modulator to the gain section of each sampler and link your knob to that. Basically there are many ways to do this.