Combine Billing document on basis of Project

Hi

I have requirement  that, Invoice should be split on basis of Project whose WBS elements are assigned to Sale orders.

We have scenario as order related billing. A) In one order there are 2 WBS elements (Item Level) which belongs to one Project X B) In second order again we have 1 WBS element (Item Level) belogs to one same project X

Now my requirement is when I do billing with above 2 different sale orders system should create one billing document on basis of Project X with 3 line item.from sales order 1 and sales order 2

Currently if I am creating any Billing document for above scenario system splitting Billing document and creating two billing document for sale order 1 and sale order 2.

please advise how i can combine Billing document on basis of Project

Thanks in advance

Regards

Pash@SD