SVN database versioning for multiple developers environment
Some of these techniques + links could help you.
From SO:
Versioning SQL Server database
Mechanisms for tracking DB schema changes
Techniques:
http://www.jilles.net/perma/2003/10/17/database-versioning-techniques/
Rails solved this exact problem by using a timestamp instead of an incrementing version number. The odds of two users creating new schema versions in the same second is pretty low.