I have an octo on mine because I'm under no false assumptions that any of my dive buddies would be able to buddy breathe a double hose.
A few months ago I was playing around, thinking of possible DH configurations for very cold water diving. So, I thought to try a Y-valve fitted with my DH (with drysuit whip) on one post and a single-hose reg (with SPG, BC inflator hose, and backup second stage) on the other. My intention was to use one of my Poseidon's. Alas, the Poseidon wouldn't quite work, so I used one of my Scubapro Mk 10 + G250/Balanced Adjustables, instead.
I never dove this configuration. However, this was a fun exercise.
rx7diver