Pick lists into one A/R Invoice

Hi,

One of our partners has 2 income orders, which see 2 prime lists. Both prime lists are acceptable for invoicing, and erstwhile we prime the 2 prime lists, SAP 'd make 2 invoices for immoderate reason. All information is the aforesaid connected the 2 income orders, lone the items differ.
What is the information for SAP to harvester the prime lists of the aforesaid customers into 1 invoice?

Thanks successful advance.

Adam