You can rewire a Boss FV-200 keyboard volume pedal to do what you want.
There's two inputs and two outputs.

I did it on mine for my guitar synth. The guitar is plugged into input one and the synth is plugged into input two. With the rocker all the way back all you get is guitar and as you rock it forward it gradually brings in the synth.

If you took one output to one amp and the other to a second amp you could pan between the two.