Why is dual crossfeeding prohibited on DA42? Diamond 42 AFM says:


  The fuel crossfeed function can be tested simultaneously with both
  engines. Proper function can be tested by running the engines for
  approx. 30 seconds with CROSSFEED selected. The operation of both
  engines with both FUEL SELECTORS in CROSSFEED position, other than for
  this test, is prohibited.


I looked at the schematics and both fuels crossfeed pipes are completely independent and separate, including the fuel return lines and heat exchangers. Fuel flow looks almost identical (other than longer pipes) when both selectors are in CROSSFEED. So why is the manual prohibiting dual CROSSFEED ?



Maybe it's not clear from the resolution, but in the original you can see on the left image, that the cross fuel flow passes through all the filters, while the cross return line passes through the heat exchanger (the blue box on the lower right side)
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.