Using different items on the items list won't change the AR account that is used for the invoices or other AR transactions.
QuickBooks Online will only use one A/R account for A/R transactions - the one it sets up automatically.
I suspect that if you open the register for the two A/R accounts in the company, only one will contain new invoices.
Also, if you create a Journal report, that you'll see the 'first' row of each invoice - the invoice total row - always uses one A/R account and not the other.
RE: The credit default can be found under the Product/Service code, but where is the default debit code for the AR in QB?
There isn't one saved on the item. The balancing account is the invoice total - unrelated to items - which uses the default A/R account.
RE: It further asks the question: why hasn't QB Online included the multi-AR functionality in their updates over the past decades, as I've seen countless requests for this functionality posted here?
QuickBooks Online is generally weaker than QuickBooks Desktop. It has many limitations that Desktop does not have. Hundreds of them. Maybe thousands.
