Never have a 50% business partner. Ever.
This is possibly the most important rule for a reason: break it, and you will fail. A 50% partner will give you a 98.8% failure rate. Having two bosses running a company together is like having two heterosexual pilots in one Formula 1 racing car: at the beginning they might enjoy the mad ride, but eventually they will hate each other's smell, sweat and breath. There is a reason why big companies, when the current CEO resigns, hire one new CEO and not two. There is a reason why there is one commander in an army.