What do I do about my demon of a cat?

He’s making life miserable for my other, docile cat (Bacio). He’s a possessive bully (Basil).

I’ve gotten another litterbox in an attempt to stop him stalking Bacio every time Bacio uses the box, but now Basil just uses both boxes. While they don’t really fight much anymore and there are moments where they lick the hell out of each other (it’s pornographic, really), there is a lot of chasing and I get the sense that Bacio just tries to find a spot where Basil won’t bother him. Basil eats both of their food, and Bacio is losing weight. Bacio is getting older now, and I worry about his well-being.

Is there anything I can do?