With the dry method you don't add water til day 18. The eggs dehydrating keep humidity around 35%. On day 18 you add water to get up to 65% to hatch. I used to have a lot of fully formed chicks die in the shell. That's why I switched to dry method. If yours all hatched fine I wouldn't switch.
I have incubated every double yolk for years trying to get twins,it has never worked. This year I'm doing the dry method for incubation to see if that works on double yolkers.