You need a tech (which I'm not), but thinking out loud, both preamps would need to be powered up all the time and output wired into the 'switch' - you need separate channels,,,,,,,,,if there is such a thing, a pedal with a 'break-make-break' type action sounds possible, but both preamps would have to be preset for the same gain signal to the main amp. Personally I'd use a multi-wafer rotary with capacitor buffers, clunky but safer, in my opinion) - over to the technical department............