How old are they? Best way to tell is by voice (females "quack", males have a quieter, raspy voice), or males have what's called a drake feather above their tail (they are curly feathers). Both of those ways depend on the age of the duck, though. You can usually voice sex by around 6 weeks old, but drake feathers won't show up until around 6 months old.
Ok, it's just too early to tell for your two right now. You'll have to wait until they start to get in their adult voices. You can't really go by size because some just grow faster or are bigger/smaller in general, and since these came from TSC there could even be a slight age difference (just a couple of days difference in age will affect their size when they are so young; ducklings grow fast).
They sure are cute, though! :)