Dan & Sam's answers are solid. I'm going to add some more context for you specific to your question. Often, solo developers don't want to go it alone but don't know where to start in finding a suitable partner to assist with marketing, biz dev, fundraising (if required) and the like.

So the fact that you have found either other and both have interest in the same space, suggest that you could be effective partners, if you can confidently convey why you are the right person to execute the non technical aspects of the business. As Dan suggests, don't go for the hard sell. It's primarily going to come down to whether you both like and trust each other, so work on establishing that common ground.

Sam is right in that if you haven't been rigorous with customer development, there are many cases in which failing to do customer development and empathetic research prior to shipping the product wil cause big problems but there are also a few exceptions to that rule.

Happy to talk to you about how to build the relationship as well as how to shape the product in a collaborative context