Thanks, TrystInn. Good to know they need to be at least a month for the quacking difference to be noticeable.With very young ducklings, you can vent check just after hatching by dilating the vent: females have a larger, cone-shape organ, makes have a narrower, but longer organ that may take several seconds to appear when dilating the vent. Past a few days, however, it gets (ironically) more difficult.
We generally sex ducks with a combination of judging the quack (males have a hoarser quack, sounds like they have a horrible respiratory infection) which happens around 1 month of age and looking for the drake feather (appears around 2 months, may take longer in larger breeds), which curls upward. The quack is VERY distinctive and always accurate, so if you can hold out until 1 month, you're good to go in sexing.
If you want to post some videos, since you only have the two ducklings, I'm sure we could help with those quacks!