Attach the levels of each amp to the pedal, one with an inverse curve. You'll need to use non-linear "curves" to get the right level when both amps are at 50%. Clarky has done some work on this because he blends from rhythm to solo in the same patch.
what I do is not quite what's being asked for here...
I don't set up two different amps with different tones and then fade between them
I set up different tones in the same amp and fade [morph] from one to the other..
this is a little more difficult to do, but personally I think it yields more natural sounding results..
if you want to fade between two different amps, there's a pile of ways to do it...
you first questions would be,
do you want them to share the same fx chain or have independent ones?
do you want the amount of gain to change as you fade from one to the other or simply set up two completed tones and crossfade between them?
so...
are you fading between amp tones? amp/cab tones? or complete fx chain tones?
one thing I would point out....
fading between two different amps will work but it will never sound as natural as tone morphing
the reason being that the higher gain tone will continue to have the same amount of gain as it gets quieter
so as the cleaner tone starts taking over, the higher gain tone will become an ever decreasing 'fizz' in the background until it's finally gone and replaced by the clean..
morphing really is the way to go...
but if you've never tried setting it up before, it does take some planning and thinking about...