Ernie Ball makes a stereo volume/pan pedal that could fade from one side to another, but it is passive and there are some concerns about signal loss by way of splitting a guitar-level signal, I don't know if that applies in your case.

I have also used the Morley A/B/Y pedal, and it was good for what it was. I thought it added a slight bit of noise to the signal compared to the chain without it. It allows you the ability to switch back and forth between two sources but also to have them both on at the same time (the EB vol pedal can do that too but the volume is lower).