DESelect Engage works on a business unit (BU) level.
Within each BU, different Contact Categories / Campaign Types / Rules can be defined.
This also means that saturation levels for a contact that exists in multiple business units are managed per BU, where the rules for each BU are respected.
In BU 1, the Sends received in BU 2 are not known. There is currently no aggregation of Sends across BUs, though this may be added in the future.