In a start-up, there's two things you're building: a product and a partnership. Sounds like you are focusing on the product only, and if you're partner isn't with you -- your start-up will fail (even if it succeeds technically).
Make sure your partner is in the trenches with you writing the business cases / user stories, writing test data / documenting of test cases, writing user manuals / making screen mockups, documenting system architecture (you sketch on whiteboard), and finally, doing manual testing early and often to give you feedback and to understand the challenges.
Later on, it'll be your job to be in the trench with your partner in the other side of the business: on sales calls, supporting client requests, presenting to investors, making hiring decisions, and establishing company culture. Don't think of it as dividing responsibility, think of it as sharing responsibility.
Make sure your partner is in the trenches with you writing the business cases / user stories, writing test data / documenting of test cases, writing user manuals / making screen mockups, documenting system architecture (you sketch on whiteboard), and finally, doing manual testing early and often to give you feedback and to understand the challenges.
Later on, it'll be your job to be in the trench with your partner in the other side of the business: on sales calls, supporting client requests, presenting to investors, making hiring decisions, and establishing company culture. Don't think of it as dividing responsibility, think of it as sharing responsibility.