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If all you want to do is to sync the users, using my multisite may be a bit over the top (and as I continously say - you WILL need a FREQUENT backup procedure to be sure that you don't mess up during the configuration. One can lose entire site contents if not used properly, or it can behave "weird" if stuff that shouldn't be shared is shared - or if stuff that could be shared is only shared partially) but that's an entirely different discussion.
What you could do is hack the user registration, removal and editing files (register.php, edituser.php, user administration - in root (2.0.x) or Profile module (2.2.x) ) so that whenever a user is added, edited or deleted, a message is sent to the other server with the new information and that database is updated.
SOAP might be handy here - but will naturally need some kind of security mechanism (password... preferably encrypted... perhaps two encrypted passwords where one depends on the other) so not just any user can be edited by anyone.
IF on the other hand, you have access to the other site's database from the first site, you could hack in the code directly, to update the other database table as well, whenever a user account is saved or deleted.
"When you can flatten entire cities at a whim, a tendency towards quiet reflection and seeing-things-from-the-other-fellow's-point-of-view is seldom necessary."
Cusix Software