In my experience this sort of setup is not at all unusal when you're dealing with sensitive data. I've implemented this sort of thing. I've also recommended this kind of solution where the security issues required it.

Keeping the more secure info on completely separate box, database, etc. protects your most sensitive data even if the first box is compromised.

While this makes implementation harder (keeping chucks of database in sync, etc.) the issue is security - not ease of implementation :-)

Whether this is sensible for your particular situation depends on the risk assessment of the info in question becoming public.... so it may well be overkill. It's not completely stupid tho'.