Very bad pic, I know - I took it with my cell about 3 weeks ago. It is of a chick who hatched and is now laying on duck eggs - the duck eggs were set to hatch a week later and here's the pic from that:
Here's another of a chick who hatched in the bator with the duck eggs - you can just make out a duck egg in the far left of the pic:
I did "lock down" my chicken eggs for 3 days without touching the duck eggs. It actually turned out to be 2 days because the chicks started hatching on day 20 - each time I took a chick out I would turn the duck eggs if I noticed a chick hadn't done that already for me.
The automatic turner would have been useful, but all my ducks survived without turning for 2 days. I don't have any experience with auto-turners, but I would imagine you would just want to be sure no chicks sit on top of the eggs and mess anything up.
I even took the duck eggs out after all chicks had hatched, set them on the counter, and quickly washed out the incubator. This all happened in about 5-10 min, though, because I didn't want the ducks to be out too long.
I made sure the humitidy was high when the chicks were hatching. IMO - It didn't matter if it increased for the ducks since they still had a week to go. Once the chicks were out, humidity went back down until it was time for the ducklings. But, if you don't want to mess around like that you could always mist the eggs due to hatch so only those are a little wet...