Ask HN: Coopetition Conflict Question

We do software development and security auditing in a very specific field. One of the top security auditing companies, which does not develop software, sometimes receives requests for both development and auditing. This company would like us to develop the software in these cases. We would pay them a percentage of our earnings for this work. The auditing company recently objected to our auditing work for customers who approached our company directly. They want us to channel our requests for auditing to them. We receive more auditing requests independently than the software development they pass along. Both companies have under 20 employees. We are interested in sharing work but not in exclusivity.

How could we negotiate an alternative win-win situation in this case? I would appreciate your views and any literature on the subject.