I don’t have an A/B switch, or an A/B/Y switch, but I do have a DoD Flanger with a stereo output. I’ve never used it with two amps though. I’ve always wanted to play with it. I have both of my tube amps at home right now, a 22 watt Fender Deluxe Reverb and a 15 watt Fender Bassbreaker. This weekend might be time to experiment a little. Two Fender amps kinda defeats the purpose, but they do sound very different so it won’t be a total loss.
Here are a couple of videos, for research purposes…
Inside the Red Head's Head 