How to support multiple mice?

I have 2 advanced trackballs and they both have button 3 button 4, etc.

Is there a way I can have separate mice or button bound per device?

Did you ever figure out a solution here? I'm trying to do the same thing, and I still can't find a dependable way to achieve this.

As far as I can tell, there is no way that BTT provides for using specific devices in different ways. All mice are the same.

Karabiner-Elements will solve your problem, however. It provides functionality that extends to specific devices. I'm pretty sure that it permits customizing even two mice of the same make and model, because it knows their different DeviceIDs. It supports mouse buttons up to Button32, so it will let you use a whole herd of mice in any crazy/wonderful way that makes your life complete.