In any startup, you ARE going to have arguments, misunderstandings and even fights with your co-founders.
You have a co-founder with you whom you know him from your school/college days. So, you can shout at him when you want to and he probably would understand. But, what in case you had met someone new and who became your co-founder over a period of time, what to do then? Do you shout ? He might get offended. Do you sugarcoat your words? It might lose its effect.
We have the same situation - 3 co-founders, 2 of them who know each other from college days and the third one is someone whom we didn’t know for a long time. The best way to deal with the situation is to TALK it out. Start with the belief that all human beings are inherently nice people, and you know what, you’ll find that it is true. So, just relax, take a deep breath and speak your heart out the way you would to a friend. And see the the effect. This obviously comes from personal experience as declared earlier.