It sounds like you'll need to come up with a custom solution - possibly a chron job that runs MySQL script to copy user and user meta tables between databases. Maybe run it twice a day? Oh and probably run a database backup on all databases prior to running it.
All I know is that you'll want a good Database Administrator to help you with this as you're going to be doing something quite complicated (multiple databases on different domains) and with a high degree of risk (as a mistake can ruin a database).