There's no direct method of transferring balances or payments between AR
accounts. (sadly, but GnuCash is not unique in this regard)
Some type of intermediary account would be required. (likely, one that
is a sub-account of AR so as not to disrupt the overall AR balance)
While not as 'clean'
I run the accounting for a member's club. Members are customers in the
accounts receivables.
Sometime a member may pay the invoices of several other members in one cash
payment. For example, in family situations where father pays membership
invoices of his children who are also members.
What I