Change migration files to use UTC timestamp instead of a number. This will solve the problem of conflicting migrations.

This does not solve the original problem, it just reduces the frequency that there will be a conflict. There is still a chance, while very rare, that two people create a yaml file with exactly the same UTC timestamp.

Please read the following: http://developingwithstyle.com/2008/05/05/the-problem-with-migrations/