It is just hard in general to hatch twins, only found one success story so far for both twins in the egg being hatched. I am still learning this is my first hatch. The problems I heard from people who attempted this varied from they are ticking time bombs and can explode compromising other eggs, one person who had an egg where one of the twins survived and one didn't, and someone who apparently started to hatch and they were so deformed she had to cull them. Because of the space in the eggs they just do not have what they need to make it, and if they get far enough to hatch they need assistance, so if you have other eggs in there and you have to keep opening to assist you risk shrink wrapping the other eggs. A lot of people told me to toss the egg early on because of either bad experiences of their own or just because of their knowledge and concern for other eggs. Humidity wise I just been doing the same thing as recommended for other ducks which everyone seems to have different ways of doing that (I did 55% first 25, then from lock down to know 65 increasing to 80 as they hatch), I have not found any specific information of incubating twin duck eggs. We have been home so able to really keep on top of keeping humidity pretty stable. Temp we kept it at 99.5 most of the time not going over 100.5....We also did not pick up egg to candle much as other eggs, my gut feeling was it was just to fragile.This is just first timer information though but hopefully it answered your question, not even sure how accurately I am doing things. Based a lot of my hatching information off this site and off the Cornell web page.