The first thing you do is buy each of them a design patterns book and start forcing them to both read, and use eachother's code. When bugs appear, don't make t

The first thing you do is buy each of them a design patterns book and start forcing them to both read, and use eachother's code. When bugs appear, don't make the author fix the bug, make one of the other guys fix it. That'll probably motivate them faster than anything that they need to 1) write common library of code with interfaces they all agree on, 2) have a version control system they all like, 3) have unit tests to make sure there's no bugs.