@Josh is correct. You can only assign one customer posting group to the customer. And if you are using sales orders, invoices and credit memos, then the customer posting group assigned to the customer is the one the document(s) will use. Josh is saying that if you make the Customer Posting Group field available to the sales order page, sales invoice page, etc, then the user can override the customer posting group to the other receivable account as desired.
I would recommend a different customer account.