sorry I don't know a thing about ducks. If you go to the Other Backyard Poultry forum you will find threads for ducks and can post on them. Duck folks should be able to give you better answers. Welcome to BYC
Welcome to BYC. Glad you decided to join our flock. I'm a chicken person and don't know much about ducks (although I've had a few), but I have to admit that ducklings are the ultimate in cute. Definitely post on our duck section and take advantage of our duck experts there. Please feel free to ask any questions you may have. We are here to help in any way we can. Good luck in getting your ducks' gender identified.
There are a couple other methods of sexing ducks, other than vent sexing. Voice, drake feathers, and color.
Females will be loud and have harsh quacking sounds. Males often have a hoarse, more raspy, quiet voice.
Mature males will develop "drake feathers", which are curled feathers on the tail.
For mallard-colored ducks (called "gray"), females will be a penciled brown while males will have green plumage on the neck and different coloring. Based on this, I think that the one on the left in the photo is a male.