An employee/user can access to multiple OU using a single login.
You will need to create two responsibilities:
Responsibility A and responsibility B.
Each one with be attached to A and B OUs respectively. Then assign both resp to your user.
You can control the OU access for the responsibility by setting up the following profile option (at resp level):
- MO: Default Operating Unit
I hope it helps,
R12 has specific functionality for this now, where one user can see data from multiple OUs. Many forms now have the legal entity and operating unit added to the top of the screen so the user can select between those he has access to. So he would see operating unit at the top of the screen, and he can choose between A and B. The Purchase Order forms have this ability.