This is a tough one. What I have found is that this usually occurs when each party thinks they are not being heard. What has worked for me is to bring everyone into a room (not TOO many, though. Find that sweet spot), give each party a turn to talk without interruption to state their case, allow parties to ask follow-up questions to each other, and then visually lay out all of the options.
A good framework that I like to use is the Value Proposition Canvas119 from Strategyzer. This paints a picture in the context of the user and can get to a product-market fit determination in a straighter way than willy-nillying it.