Well, several problems face you. The first being of course that you don't know when you set the eggs so you don't know when they're due. That you may be able to solve by candling them and comparing them to candling pictures to guess what day they're on.
The other issue is that ducks and chickens don't have the same incubation period. Ducks take 28 days to hatch and chickens 21. That means that at day 18, your chickens are going to need to go into lockdown stop being turned, and have the humidity increased for hatch. Meanwhile, the ducks are going to still need to be turned to keep their chorio-allantoic membranes growing properly, and will still need a lower humidity so that they can keep losing moisture and their air cells can develop as they should.
Really the only way to solve the second problem is to buy another incubator, or try to deal with it this time and hope the ducks hatch okay and not do staggered hatches again in the future.
