This happened recently.
It apparently wasn’t due to empty tanks though because the report says “Draining the tank under the control of firefighters at Brie / Champniers aerodrome.”.
Unusual method of recovery too – rarely done.
There are multiple single point failure scenarios like a clogged main tank or fuel related problems – bad fuel, bad filters or other fuel distribution errors.
Considering the low resale values of 337’s, I’d probably just have left it there.. 
Not that I am saying this is the explanation, but if the tanks only had unusable fuel remaining, they would still need draining.
AdamFrisch wrote:
There are multiple single point failure scenarios like a clogged main tank
The 337 does not have a "main tank.
It’s bog simple 1 tank for the front engine and another for the rear.
http://www.337skymaster.com/messages/archive/index.php/t-2325.html
This is the famous double engine failure over the Atlantic.
They may have drained the tanks, but it does not say how much fuel was recovered, ie. if it was only “unusable” fuel. It also does not say what tanks the fuel was recovered from; mains or auxiliaries.
How many probable causes might there be for two engines quitting simultaneously other than fuel mismanagement of some sorts?
Any fuel system slightly more complex than say a PA28 with a left/right selector, does unfortunately have gotchas or caveats.
