After 'Yachtgate' you wouldn't have expected these two to have much to say to each other. Yet it would appear that they share common DNA over the matter of the EU. Peter Oborne makes some interesting observations (full article can be found HERE), best summed up in this short paragraph:
"Europe is another common area of interest, for both men are ardent supporters of the EU. Mandelson is the mastermind of the campaign to keep Britain in Europe, while Osborne is the Government’s official negotiator, charged with obtaining better membership terms."
Somehow, the concept is not surprising - but it goes to demonstrate that the EU project goes well beyond party politics.
For further insight on the matter - see THIS from eureferendum.com