Hook up Send from Valvestate into Return on Fender. Keep return on Fender at -11dB to start with. Start with volumes very low on both amps. You don't want to destroy anything.
You need to turn the Fender channel volume to zero. It can't be used when accessing the return only on the Fender because the Fender's preamp section is cut off. If it's not cut off, turn it down to zero anyway.
If you want to have both Fender and Marshall, use a 2ch effects looper pedal. Hook up like this:
1. Guitar>Input of looper
2. Fender send>ch1 return of pedal
3. ch1 send from pedal>Fender input (front of amp)
4. Marshall send>ch2 return of pedal
5. ch2 send from pedal>Marshall input (front of amp)
6. Output of looper>Fender return
A picture may explain a thousand words. Here's part of my setup below:
You may or may not get a pop when switching. I don't have an issue with my setup above.
In fact the looper pedal above is a parallel
loop pedal that allows me to blend and adjust individually the volume outputs of both the Mesa Recto and any other preamp connected to ch1. Works like a bloody charm. I can also send to the output of the looper an unaffected clean guitar signal through either ch1 and/or ch2. How cool is that?
I have 4 rack mount preamps and two valve drive pedals that I still need to hook up to test. At the moment, I don't see an issue.
One thing you need to know if you think the parallel idea is good; one of the loop channels NEEDS to have a phase switch. This is required because you don't know if a preamp's output, hooked up in this method, will be in or out of phase with your amps preamp. If it's out of phase, it'll sound weak and gutless. Both need to be in phase.
Hope it's enough info for you. Keep on rockin'....
EDIT: By the way, I can switch the above loop pedal to have both on or one loop at a time or bypass completely to have only the guitar signal going to the power amp > ***super clean***.