In my work at the State fostering emerging technology based spinouts, I've had a similar conversation several times in the past few days about what to do if a founder relationship melts down.

This can be a terrible problem for entrepreneurs.  Sometimes founders diverge in their view of the business strategy, but many times they diverge in their appetite for risk.   Entrepreneurship is hard work, and it entails risk.  Calculated risk, yes, but risk anyway.  Sometimes a founder refuses to put themselves on the line for the startup - even though they want that big chunk of founders shares.

This is particularly important in university-centric spinouts. Professors often don't realize that "running" a startup is a full time job- and that they can't do that AND have a full time obligation at the university.  The Feds are explicit about this in their SBIR program, where they say that the "PI" at the startup cannot also be employed full time at the university.  This should also apply to startups.  Either a professor has to leave academia (which I rarely recommend when I work with these teams) OR they have to take a lessor role such as Scientific Advisor, Director or other part time role.  With this lessor role comes lessor founders stock.  That's right, you don't get 50% of the company because you've been paid by the university to do the research and you're not taking the personal risk! 

That all said, it is essential for founders of ALL kinds to have a buy-sell agreement.  Agree up front on the terms upon which one partner (or the company) can buy back the other founder(s) if there is a "parting of the ways".  We were so fortunate in our founding team - we had strong shared values and we'd vetted those values in advance and we all stayed coherent in our vision for the company.  However, more often than not, at least one of the founding team will need to step away from the company.  As one of my Directors said, when things "go wrong" if everyone behaves gracefully, the relationships can still stay intact.