I'm going through a breakup right now with my partner, where he and I started the company as 50/50 owners and then later brought on a third party which was then split 38/38/24 between us. This was a mistake for many reasons, which I won't get into within the scope of this forum. Ultimately, I (and a friend that was helping me for free) ended up doing 98% of the work over a year and a half period and now my partners stand to benefit from our tireless efforts.
When my partner and I first conceptualized the idea, some five years ago, we naively had no founder agreement, no vesting agreement, no milestones, and no reason to think that we would ever be enemies. Flash forward to present day and we're faced with going through mediation (or worse) to decide what is the most equitable way to split up the assets (that I created) and move on.
If I took anything from this lesson it's that you ALWAYS need to plan for the worst case scenario. No matter how peachy things are in the beginning, when creative juices are flowing and you're wrapped up in the excitement of the moment, the unfortunate reality is that at some point you're going to butt heads. In my case, our vision of what the end product should represent somehow went astray; once you reach that realization there's no going back. At that point you've surpassed a simple lover's quarrel and reached a very definitive fork in the road. In our case, neither one of us was willing to compromise our vision so we each decided to take our own path. As relieved as I am to be free from the burden of carrying them on my back, I'm now faced with potentially losing a year's worth of work.
Take home: if, and only IF, you feel the need to maintain equal ownership then you absolutely need a safeguard in place. A third party (or better yet, a Board) is always a good idea, provided they see your vision and can offer value during a conflict. In my case, our third party was a financially vested partner who is a contract attorney and, the real kicker, was also childhood friends with my founding partner. Let me state the obvious here: bad idea. Do yourself a favor and learn from my idiocy — I know I have.