Really? I figured it was no more confusing than not being able to stack std cannon on a medium or something else like that. Oh well, I'll whip up a quick two slot icon.
The idea behind not being able to stack Cannons is that there're only two barrels, so how can you fire multiple shells? Of course, this was the same reasoning for not letting players stack ML's, but that went out the window when people remembered how cool shotgun heavies were. One crazy idea for making other tanks more versatile is to remove stacking of ML's. Right now, one of the issues of trying to have proper LT and Medium research is that researching an awesome LT ML would just have people stick 5x of them on Heavies and make the ML in question OP as shit. By removing weapon stacking, you can have single-slot weapons that are relatively good for LT's but relatively shit for heavies.
You know what happens when I see someone tangentially support something that I've long been considering? I'm weak, man, I'm weak.
Kinda, I don't think you can put a cannon on the other slot. So it is more of a hard cap of 2 cannons, not 2 each which is a different thing altogether.