How do assign the differnt Banks to differnt responsibility but OU is single in R12?

We need to assign different user to make payment on basis of bank. For Example, we have five banks (A,B,C,D,E). "A" bank will assign to user 1 to access "A" bank check numbers. "B" bank will assign to user 2 to access "B" bank check numbers. Similarly, others are also.

Is there any option available in R12? If it is there, how do? Please.