From the Administration Manual for PBCS; -
Planning Artifacts That Can Be Assigned Access You can assign access permissions to:
Dimensions, including user-defined dimensions. Assign access permissions to members by selecting the dimension property Apply Security. If you omit or clear the Apply Security setting, all users can access the dimension's members. By default, the Account, Entity, Scenario, and Version dimensions are enabled for access permissions. Optionally, access can be assigned to Year and Periods.
Launch privileges to business rules
Form folders and business rule folders
So, no, there is no cube security level.
But, if you have a different top level member for both cubes in a secured dimension then you could assign 'no read' descendants to remove access to the entire cube.
Or if you already have different dimensionality between the two cubes then apply your top level security to one of the dimensions that is different.
Hi, I believe you are working on standalone essbase application if yes.... you can leverage "Essbase Filters" to have the security at dimension levels.
Are you setting up security via Shared Services?