Building trust is one component, but it's equally important to explore whether you're compatible with your cofounder (or SO) across a wide range of emotions and circumstances. There are people I personally know pretty well who I trust but don't think I'm very compatible with. They might be great advisors or board members, but I wouldn't want them as a cofounder.

Like you said, in a constrained environment like the military, it might be possible to achieve this in a short period of time through deliberate actions. I'm not sure how much that approach would prepare potential cofounders, since they're going to be operating in a much more ambiguous and freeform environment.