Two different versions of the same base theme in the same site
Themes, like modules, are identified by their names. The version number is never really used (except for updates obviously).

Anyway, i found myself facing a similar problem sometimes, and running a find&amp;replace on the theme directory to change every occurrence of the name with another inside the files never lead to unexpected behaviors.

well, i also had to manually rename the .info file, so it's not a completely automatic procedure, but still nothing that caused problems of any sort.