AFAIU, the delegation works at invoice level - which of course makes sense. I’d rather need it at the level of subscriptions (person A is the subscriber, person B pays for it). So I guess, it’s easier and more consistent to merge those two persons.
EG
Subscriptions generate invoices so you can manage the delegation after the subscription.
Another option is to use the invoice_party field of the subscription to generate the invoice to a diferent party than the one of the suscription.
invoice_party