Depends a lot on the exact boundary for the split... if it's a simple 'these tables are billing, these tables are customer accounts', then why not use two different DBs? The customer webserver only has access to the DB with customer-modifiable data, and the administrator's webserver has access to both DBs.

Couple of different ways to split it, based on paranoia level:
- two separate database servers on physically (and logically, eg. network-wise!) isolated machines
- two servers on the same machine, which is only marginally better than
- two users on the same server, with permissions set appropriately

Trying to keep a copy of the customer account data in the administrative DB as well seems unnecessary. It won't kill you to have the administrative interface dealing with two different databases.

Hope this helps...

Jeremy Weatherford
xidus@xidus.net
http://xidus.net

I guess I should register here eventually...