I have a thought that I think would be pretty easy to test. I have been working quite a bit with the different options and configurations and Dirsync and have a good handle on breaking and fixing it.
Prior to the very latest version (June 21,2013), you could set a scope on DIRSYNC by running MIICLIENT.EXE, which is part of the dirsync binaries. I know from experience that once you create a user in Office365 but the link gets broken, it just sits there and is ignored by Dirsync. (You can clean up these accounts using powershell.) I other words, I have seen where a user is left unmanaged by DirSYNC and is basically ignored.
My thought is that you should be able to set up 2 DIRSYNC servers, with non-overlapping scopes, and have different DIRSYNC configurations for the 2. Password Sync for one, not password sync for the other.
Not supported I am sure, but based on the scoping, should function.