I know this exists, I just don't know what to look for that will do the job, perform well and be a good price.
I change between guitars during a gig, depending on the song we're playing and at the moment I unplug one guitar and then transfer the wire into the new one. I'd rather have both guitars plugged into a pedal at the start of my chain so I can simply press a button to determine which guitar is active. I'm guessing this is called something like an A/B box or A/B/Y box as I seem to recall reading about them before but I can't find such a product at the moment as I'm not sure what it's actually called or who makes them.