In Manage Multiple Customers Contract Sales, it states that "Product-based contract lines are automatically associated to the primary customer". This makes no sense in the context of allowing multiple customers. The idea would be that one entity is quoted and signs off but another (e.g. a purchase to pay services company) receives the Invoice.

URL is https://learn.microsoft.com/en-us/dynamics365/project-operations/pro/sales/manage-multiple-customers-contract-sales