You talk about it. Either you reach a decision you both agree on or you don't. In the latter case, it's usually more important to keep the ship sailing than to have it your way, so one of you will have to give up. If you both can't, agree on a fair procedure - like asking a third party or throwing a coin - and stick with the answer. If you disagree with the decision, you can always say your disagreement explicitly and refuse to take responsibility for it, but that's totally orthogonal to supporting it when the decision has been made.