Coceptualizing Project Org. Hierarchy

Hi,

Would really appreciate if somebody can share their inputs on the below scenario we are faced with:

We have four Business Units each classified as Project Expenditure/Event Organization and have multiple OUs each classified as Project Task Owning Organization. Each OU can perform work for multiple BUs.

Hence, From OU responsibility expenses can be entered for any BU. So we require all BUs to appear in the LOV for expenses org.

In the light of above requirement how to design Project Hierarchy. When I'm trying to include an OU as subordinate to more than one BU, it is not allowing.