Hi Kari,
The screenshots which you have given on 22-Mar-2013 was different. In that you had 2 different partners in split analysis. In that it was also showing 2 different billing dates, different tax classifications for the customers,different VAT registration numbers.
Now, in the recent screenshot(the above one) are you still getting 2 different Invoices? If so, i request you to share the split screenshot.
Regards, Sai Krishna.