Wish to Use Common Reserve for Encumbrance Account Incorrect

Summary:

We have 2 BUs e.g. A and B. Both have their own bank accounts, but use A's bank account for payables.

A's Bank Account: 1-10-A000-11010

B's Bank Account: 2-10-B000-11010

Only A's BU is enabled for budgetary control and encumbrance accounting since B doesn't require it. The way invoices and payments clear work properly for B, i.e. they flow to A's accounts payable control and A's bank account.

The problem:

Staff in BU A sometimes create requisitions/POs that crossover to B as they manage projects on their behalf. As the staff belong to BU A, our managers are saying they should be selecting BU A as their procurement BU despite the budget belonging to BU B.