Hi,
i know from seagull php framework
http://www.seagullproject.org (which i use a lot), that it's possible.
The User manager has some observers (similar to plugins?), that can do syncing of user e.g. to FudForum. Code can be found in Seagulls User Module (RegisterMgr, PasswordMgr and the observer classes).
Of course you still have two seperate logins for blog and gallery; maybe you can disable changing of user settings in gallery, so you only can register or change passwords using s9y, which again with a plugin (or observer) which syncs the gallery user DB.
I don't know if there are some event hooks in s9y available for this...
If you need any further information or help please contact me
Another possibility could be (if both support it) either HTTP-Auth or LDAP
just my 2c
Werner