The same question was asked of Dr Chris Ashton of the Indian Runner Duck Association and this was her reply
"You are correct in thinking that 2 males are bad news in a group, but one is OK. Generally, the females are happier and more confident if there is one male around.
Occasionally, even one male can get obsessive over a female, but that is fairly rare. On balance, I'd keep one male. But if they are re-homed as 2 drakes for slug hunting, that might work out easier for you.
best wishes from
Chris"