You can use the Mark's effects loop 'send' into the input of the V-Twin. There's no need to connect the V-Twin 'out' to the Mark's effects loop 'return' if you don't want to. I'm assuming that the Mark won't have any output because the effects loop 'return' is not used (the Mark's preamp is now isolated from it's power section)
Set the V-Twin to clean channel. The V-Twin will most likely colour the sound (I'm assuming this) as it will add it's own tone. It is a preamp after all.
Just watch out for the channel volume coming from the Mark into the V-Twin. If it's too high you'll distort the input side of the V-Twin.... *maybe* even do damage if it's too high, so start with a low channel volume from the Mark.
Plug headphones into V-Twin. Use good studio headphones (AKG make good ones for a fair price) if you want the best 'flattest response' sound possible. DJ headphones and HiFi headphones will colour your sound. Keep the Master output volume of the Mark at zero. You don't need to drive your speaker/s if you're using headphones.
With this setup you should also be able to blend the Mark's channels and V-Twin's channels to get different tonal flavours and further 'multi-channels'.
Without headphones:
Plug the V-Twin 'output' into the Mark's effects loop 'return'. Set the V-Twin's gain, Bass/Mid/Treble and master output volume to taste. Adjust Mark's master output volume to drive your speaker.
A word of caution..... DO NOT have the speaker/s of the Mark disconnected, regardless if you use headphones or not. You will damage the output transformer and you'll be up for $$$. Make sure your speaker is always plugged in before powering up.
