Firing a cofounder who owns more equity that you

TL/DR, with all the weird "little slice people" references removed:

You ask the co-founder nicely: give them two warnings, and then ask them to resign if they don't improve. But if they refuse, you leave the company yourself.

(You can also read the book author advertises so you don't have to feel bad about asking cofounder to leave, or leaving the company if they refuse)