I realize what I'm gonna ask is a fringe case, in fact I'd be surprised if it ever came up during games at all...

Let's say I have two units and Hypnotize. As a side action I give Bypass to one of my units, then as a main action I have both of them attack an enemy unit.
Now the rules state that when a Phoenixborn guards they don't guard a single unit - they take the brunt of the attack. But one unit could be legally guarded against, the other could not. So what happens?