If you separate boys from girls, a fight may ensue. Spring is a tricky time anyway (all those duck hormones!) There is a way to sex ducklings, but you would need a professional such as a vet to do it.
My two boys are separated from each other right now because they are bullying each other for the one girl duck. I was fortunate when I got the ducklings in that I had two of each. But since my Daisy was killed by the turtle, there's only one female for two boys now.
I'm considering getting another female and introducing her to the three remaining so maybe they can all pair up again, but I'd want an adult female.
I'm really nothing more than a "backyard expert" at best when it comes to ducks. I knew absolutely nothing about them when I purchased 4 little fuzzy yellow things a couple of years ago. I just watch their behavior and provide food, shelter and plenty of love.
If you do decide to separate the boys from girls, I would definitely observe them carefully for a while to make sure they don't start fighting.
Good Luck!
GrammaDuck