We worked together for two years, split six months ago. We didn't fight but the process took two months of my time and was unnecessarily expensive. I'm writing so you don't make the same mistake.
We split shares fifty-fifty at the start and said "no problems will arise between us." No problems did arise; we just started wanting different things. That's also a problem but there was no provision for it in the contract.
Three things that cost us dearly because we didn't put them in writing: no vesting schedule, meaning we owned 50% from day one. It wasn't clear what happens to equity if someone leaves. And decision-making authority wasn't defined, so both of us could decide everything.
We eventually agreed, but we're lucky we resolved it amicably. If we hadn't, the company would've been deadlocked.