This might be a bit difficult to explain but here goes. Alongside my bass I also use a Mellotron m4000D. I put them both through my bass amp via a Behringer Xenyx 802 mixer because the amp only has one input. But sometimes I'd like to put the bass straight through to the amp. Without unplugging anything. preferably. So I want a double a/b switch as it were - when I press it I want to switch out the mixer input AND output and go straight to the amp. I could do it with two stomp box A/B switchers but that's two devices muddying the path when I want maximum clarity.