That's a sad experience but not rare unfortunately.
What you are going through is very common, two people get their similar skillsets together and develop a product with no clue for preparations into what the future will look like for the company and for them individually and as a team. I would not necessarily quit but revisit the contracts and agreements that you must have had in place and maybe try creating new positions for the both of you where is the a middle ground, add a third person to be liason and conflict resolution and in charge of what you two clash the most. Focus on what strategies your company can rely on, and what each of you are good at and happy to do. Passion can come and go depending on your mood and situation so don't make judgement or permanent calls based on what you think you are passionate about that moment. You might think is not a moment thing, but you make decisions based on how you feel at any give moment. Whatever you do, working with another company you will be working for another person.
Is it lack of respect that you have? Maybe you don't enjoy working with the person so consider who is better at the current roles and determine what leadership changes are needed, maybe both need to step down. Take your board into consideration and offer them a job or hire from outside. Become an evangelist for your product.