You can but often both require different humidity conditions. I have done it ok depending on the breed of chicken and duck you are hatching, and I generally used the dry incubation method which is good for thick shelled eggs, but not so good for poor shelled eggs.
If the eggs are similar size and similar shell thickness then have a go. Remember that ducks take around 28 days and hens 21, but I have always found it depends more on the size of the egg rather than the type of fowl.
Eggs wont begin to develop until the hatching temp had been reached at the centre of the egg. So larger eggs take longer to warm and are therefore slower to hatch.
You may have more success if you have an incubator and a separate hatcher. So the eggs are moved to the hatcher about 3 days before the hatch is due, then you wont need to time the eggs so precisely.