That's where you trust that both parties will fight hard for what they most want. If you fight as hard as you can for your terms, and they fight as hard as they can for their terms, then presumably all the issues will be laid out on the table and the agreement simply won't get signed unless it's as good as it can be.
It's the same reason that adversarial systems law, economics, international relations, etc. tend to work. People have more information about their own desires than that of others, so if you task each person with looking out for his own interests but ensure that they also respect the clearly-expressed interests of others, you end up bringing as much information to bear as you can.
It's also why systems like law, economics, and international relations fail. People don't care about issues which only marginally effect them, so they don't bother.
Take pork barrelling (edit - I used a different example, but it would derail the thread). Nobody (at least, nobody who's notable for anything except being anti-pork) fights to limit it, because no-one loses a huge amount. But a small minority of wealthy vested interests do stand to gain a lot.
But your point is basically correct - the systems mostly work, because people's big issues get dealt with.
It's the same reason that adversarial systems law, economics, international relations, etc. tend to work. People have more information about their own desires than that of others, so if you task each person with looking out for his own interests but ensure that they also respect the clearly-expressed interests of others, you end up bringing as much information to bear as you can.