I'd say you can. However I'd bump your humidity to around 55% for ducks and geese. A lot of people say 65% but in my experience that is too high. Aim for 55%, no higher than 60%, up until day 26, and them bump it up to 65%-75% for hatching. Also, I highly suggest spraying them with clean, warm water (warmer than the eggs or at least as warm--if your incubator is large, try to keep the spray bottle in the incubator) starting at day 7. I personally don't start at day 4. Spray once daily, don't spray too much, and don't cool them afterwards. I've found no advantage in cooling them and some people forget to put the top back on the incubator so there's risk involved. I think if you set the goose eggs two days before the duck eggs, you'll be fine if you stop turning at day 26... As that would be day 24 for ducks. See, I stop turning goose eggs very early (as in, I turn for day 25 but stop ON day 26), but I stop turning duck eggs on day 25. I think for a compromise day 26 would work.
Edited by adrian - 3/19/11 at 4:48pm
As for the dip test... I've never done that myself. Is that an attempt to assess viability? If so, just candle. Invest in a good flashlight and learn to assess eggs even if their shells are thick, like goose eggs are. Duck eggs, you'll have no trouble with. A few days prior to hatching, you will likely see something akin to this in goose eggs:
In duck eggs, similar, but you'll be able to see more.
If the dip test is not to assess fertility but just to get the eggs wet, just spray them! I think it's much more natural. Stop spraying when you stop turning.