If your technical decisions are black or white, you work on trivial problems - collaborate to tackle a bigger issue where you need to consider tradeoffs.

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.

How do know there is such a thing as a technical decision which is black or white?

Like this:

http://www.fao.org/docrep/008/a0032e/a0032e0w.jpg

Your technical preference is a position you take to achieve your interests. To collaborate, you have to be willing to reveal and effectively articulate your underlying interests.