Error when inserting more than one customer in the contract

We are creating a contract with buy intent and we need to enter two business units as customers in the parties session. But when we try to enter the second customer, the system gives this error message:

The contract can only have one party with a party role source of business unit. (OKC-195721)

Have you gone through this? Is it possible to enter more than one business unit as a customer in the Parties session?

Thanks,

Cezar Mattos