Switching subject matters that you are working on is great for relieving mental fatigue. That was my study trick in college: whenever I found my mind wandering while doing homework for one subject, I would switch to another subject. I had read somewhere some research finding that mental fatigue is largely subject-specific, and that seems to be born out by my experience.

In the hacker context, if coding is going hard, switch to business promotion for a while, or design, or whatever other task you perform in your start-up. Advise your co-founder to switch whenever one task seems too fatiguing.