Hi SC
Cant you think of replacing your document type for the 2nd posting with the help of a substitution or BTE?
If that is not suitable, you will have to copy the Business Transaction Variant 0100 to Z100 and add Vendor in that. You would then assign Z100 to doc type AB
You would go to the SPRO node "Extended Document Splitting" and then copy 0100 to Z100. Ofcourse, it is a bit of trial and error
Rgds
Ajay M