Why is dual crossfeeding prohibited on DA42?
As I also could find no satisfactory explanation within the diagrams, AFM, POH or Maintenance Manual, I sent an email to the technicians at Diamond Aircraft.

This was their response, so far:


  Hi Chris, I can only assume that if this procedure became too routine, eventually a pilot would select the second electric boost pump on with both systems in cross feed, in flight.  The high pressure created would damage fuel lines and possibly damage the HPPs for both engines.  That would not be a good day.  You can see the step prior to going to cross feed, Fuel Pumps check Off.  I have asked Austria for an explanation but I think I am close to the mark.  
  
  Customer Support,
  Technical Services Specialist  


Please note, that the customer service technician actually did include his contact information, but that I have removed it for privacy.