Often startups have more than one founder and while it is possible that one of the founders is more equal than the others, nurturing the relationship with a co-founder with tact requires close attention. Know what could be possible stress points and, better still, ensure that things remain on an even keel throughout the life of your venture.
Communication is crucial
It is important to correspond and keep each other posted on all decisions taken, even if it pertains to the other's area of competence. Say, if my competence is marketing, I would ensure that I discuss the advertising campaign with my co-founder and appreciate his feedback, rather than springing it as a surprise on him, though I am well within my rights to approve it.