1. Absolute trust in each other
This is rightly set as the #1 condition since without it disaster may well occur at some time in the future. One Friend I have said that this meant you would be comfortable in leaving your wallet (or handbag) for safe keeping with your partner. I think that is an excellent test of your trust. The ultimate condition here is that such trust will be durable even if circumstances go away.
2. Complementary skills
If you both have the same skill set, then the reason for the partnership is somewhat weaker. When skills are complementary, much richer creativity is possible and you have the benefit of different points of view.
3. Compatible working habits
Nothing will sap motivation faster than feeling your partner is not pulling his/her weight. You do not need to follow the same work schedule and indeed different work schedules may give better coverage to satisfy customer needs. Each partner should feel that there is a fair division of labor.
4. Good communication
Nothing will erode that absolute trust faster than a failure to communicate important news or decisions. With modern technology and smart phones there is no excuse for not staying in touch on important issues.