Melissa,
I am going to think "out loud" here and hope something will be helpful.
Maybe it's a coincidence you've had two ducks affected by prolapse so close together in time. If that's the case, it could be laying problems. Sometimes prolific layers get prolapses, sometimes oviduct...
I'll be turning in soon, but would like to ask you to describe their environment, what they are fed, what other animals are around, how secure their night pen is, how many drakes, how many ducks, things like that.
Very sorry for your loss.