Common or separate database when online

We are developing web extensions for an existing client – server enterprise solution. The main enterprise data resides in Oracle. This data has to be protected as it involves privacy issues. The web extensions however require the data to be current at all times.
If we want to maintain 2 separate databases ( one for online and one for the client – server access) – what methods can we use to synchronise them? Secondly, what security steps can we take in case we decide to use a single database for both applications?