Dual separate registrations needed for same forum.

[Marco van Herwaarden]

You could even use Public Joinable Groups with a Group Leader assigned to make this work.

Create a Joinable Group for your customers. Public Joinable Groups are normal Usergroups, with the exception that user can join them themself. If you assign a Group Leader all requests to the group must first be aproved by one of the Group Leaders.

Think that is the best solution for you. Make them fill out purchase details when they apply for the group.