If you have customers that need to be invoiced together, merge those customers together into a billing group.
If you have customers that need to be invoiced together, merge those customers together into a billing group. Any information applied to a billing group will take precedence over the same information applied to the billing group's individual customers, in regards to the billing system.
Treat billing groups as customers, as billing groups have their own invoice and payment history separate from the individual customers that make up the billing group. If any invoices or payments are made for the billing group, those invoices and payments will forever be attached to the billing group, and will never be applied to the individual customers that may one day be removed from the billing group.