We have assigned beds in our bedroom, too. Didn't start out that way, but DH and I were woken up by snarling in the night. Sahara had the better bed, and Coltrane was looming over her, trying to squeeze her out.
The fact that Brilly's aggression only shows up when Wabi is in the better bed makes me think this is the cause, not a sleep startle. He also has been picked on by Pogo, right? Being closer to Pogo might be adding to his stress, making him more likely to go after Wabi. Good luck with with rearranging your room!