Currently if a customer has multiple bill to addresses (purpose = Invoice), user can select the secondary invoice address on Free text invoice header. However, this capability is not available on the Sales order header. The work around is to update the default invoice address on the customer record temporarily, generate the sales order invoice that needs to be billed to the secondary invoice address, and return to the customer record to update the default invoice address back to the primary one. This work around will not be feasible if there's high volume of invoice that needs secondary bill to address.