I'm not experienced with hatching duck eggs. I've only incubated chickens and only had to assist one, this last time I hatched.
How long have they been in the incubator?
If it's been 3 days since that one pipped, personally I would start to help it. Just keep the membrane moist and watch for bleeding.
I just read on here that it can take up to 48 hrs for ducks to hatch out all the way. However, there is a wonderful guide to assisted hatching on here as well. I followed it and my little chick made it.
I'd wait until a few more experienced duck people chime in, but when it came to assisting my chick, i went with my gut and decided to help him. I didnt help another when I thought I should have and it did not make it.
Good luck with your new babies!