Hello--I can't speak for chicken eggs, but the duck egg info you've gotten so far is correct. Humidity should be around 65% and then going up to 80% by the time of hatching. It will be difficult to hatch them together, but it wouldn't be the first time someone has done it. Conditions don't have to be ideal to be good enough.
I would put the duck eggs in a week early if you can afford to wait to put the chicken eggs in, that way they will hatch together. Otherwise, you're either going to have ducks eggs sitting in humidity that's too high (so the air cell can't develop properly) during the chicken hatch, or you're going to have chickens stuck to their shells because the humidity is too low.
However, ducks like higher humidity in the bator anyway, so it probably won't hurt them too much to have a few days of higher humidity while the chicks hatch, especially since overall your humidity will be lower to accommodate the chicken eggs--so it's probably something of a wash.
You will have trouble cleaning the incubator though, with the duck eggs still in there--and believe me, you will want to clean it--it will be gross after the chicken hatch, and all that bacteria and humidity adds up to a breeding ground for illness for the unhatched ducks.
The down side to putting the duck eggs in a week early is that means the chicken eggs sit for a week. Only do this if you know they are extremely fresh--every week that they sit outside the bator their hatchability will go down.
So, there are pros and cons either way, and either method will take some special care. I hope you have excellent results!
