To debatch this will require a 2 stage process. The first requires grouping the orders and the second will debtach them.
To group Xml records using some arbitary content, requires a custom Xslt. The best technique is Muenchian grouping (google Biztalk and Muenchian for examples).
You can then pass this thru another pipeline to debatch (the output of the first map should be a envelope/document schema) or debatch using a loop inside an orchestration.