A passive ABY can be used either way. 2 inputs to 1 output or 1 input to 2 outputs. You could probably add a pot/pots to the box for volume control but you do have volume control on the guitars.
I think that will just load down the signal. A long time ago (lots of those in my life at this point) I took lessons with a guy and we both plugged into the inputs on his Fender amp, my guitar had EMGs and he had passives (a Duncan Holdsworth IIRC) and my EMG would walk all over his passives because of the impedance mismatch.
So, I don't know, and A/B will work fine for switching the effect or whatever is plugged into them to the output, it's when you try to mix them that things get weird. So, what are you trying to mix?