If your "ideas man" has fully committed to doing all the sales/marketing and the dance of raising capital, leaving you to work on the product, I think that's a very fair division of labour. After all, a finished product is almost as worthless as an unexecuted idea if no one ever actually purchases or uses said product. I say almost because, in theory, your product could have enough "obvious" value on its own that you could just sell it to another company and exit comfortably.