Apply return from one customer to another using national accounts

[Luvsql]

We do episodic series. What happens often is we may need to credit Series 1 at a later date, but apply that return to another customer (Series 2 which is a different customer ID). I assumed national accounts would allow us to do this, however, when I choose Series 1 customer in the apply information, even though the Series 2 customer is setup under the same national account, I cannot apply the return from 1 customer to another.

Why can't this be done?