It really depends on the kind of product you're building. In certain situations, non-technical co-founders will have more work on their plate than the engineer

It really depends on the kind of product you're building. In certain situations, non-technical co-founders will have more work on their plate than the engineers. In other situations, they'll have very little to do. It sounds like you're building the kind of product where engineering is most of the work. But it's still worth asking yourself: is engineering going to end up giving you a real competitive edge in your market? Or is it the marketing, operations, sales, or other back-of-house departments that will drive your business? If the former, maybe you don't need the founder. If the latter, than his might be the most important role.