Split data across several databases for security or not?

Hello there!

First, I’m pretty new to Mongodb. I’m also not a programmer, but I have a basic understanding of programming and Javascript.

Somewhere I have read that it is advisable, for security reasons, to split a users data across multiple databases.

Is that correct or is it equally secure to just have the data on one place and to carefully set the role permissions?

I’m making a social app. My idea currently was to use 2 databases:

1) ClientDB (For any requests a Client app could make)
1a) PrivateUserData (Collection which includes only data the user itself can access. Like settings, messages etc.)
1b) PublicUserData (Collection which includes all publicly visible data like username, last sign in etc.)

2) ServerDB (Only used internally. Includes Account data, purchases etc. and can not be read or modified from a clients side.)

Does this make any sense or is it way too slow or redundant to separate the data?