I have not read the entire thread so apologies if I am covering what has already been said but...
Airbus have warned of a situation that could indeed lead to a 'Dual HYD Failure'. The circumstances were a leak in the Green system after start that was not picked up all the way to Take-Off. On T/O the leak rate increased and total GRN system loss ensued but the warnings are inhibited because of flight phase. As has been pointed out before, the Yellow system began to drive the PTU, but with no fluid to drive in the GRN system it spins at an almighty rate and very quickly overheats the Yellow system. The aircraft now climbs above 1500' and lo and behold a Green system failure and Yellow 'Failure'! Of course the Ylw is actually an overheat that could be reinstated once cooled, but from memory the crew didn't bother with that they just went straight back to land.
In similar circumstances once the Ylw fluid had cooled and the PTU selected off the yellow could have been re-instated.
Oh yes another thing...
An Airbus rep told me of instances of Green to Yellow transfer but it is not 'intentional'. It occurs in the brake shuttle valves (Green / Yellow system) perhaps when seals fail internally? Sorry can't remember the exact details now.
Happy New Year to all.