Hi @wocair 

This Q was posted a while ago, but it is on the most viewed list... 

This is how I handle it...

Create a new client (Say Mr. X) as the main client, then the two sites (say company Alpha and company Beta) as the two sub-accounts. You can then issue a statement for Mr. X with the invoices for the two sites. It will only show the invoice numbers though and not for which company/site it was. (But you can always add the two sites statements to that one... just a thought.)

Create a new customer Mr. X.

For the sites. Edit customer, tick the box "is sub-customer", then select Mr. X. 

I hope this helps.