[TYPO3] how to log into more domains at once in a multi site typo3?

[Claus Faber]

Hello list,

I have several (sub)domains in my typ3 installation. More than one of 
them require a front end login. We have the same users on all domains 
(and we want it that way).

My problem is, that typ3 requires _each_ domain to be logged in 
separately. If I log in on one domain, the login is invalid for the 
others.

Is there any way to teach typ3 to log fe users into the whole typo3 
instance at once? (I didn't find any documentation about this problem)

thanks,

Claus