The duckings and the mother are unaware that they are a family. You have to treat them as though they are just strangers that need to form a flock. When mixing two groups of ducks there can be a bit of fighting, but it usually isn't serious. However, ducklings are too young for that, so you need to wait until they are older. At 8 weeks you should have no problem. You can probably combine them a couple of weeks earlier, just monitor the situation and be quick to separate them again if need be. One of the best things to do is to keep them separated, but visible to each other.
If both of the ducklings turn out to be males, you will want to get rid of them. They will fight each other viciously to earn the right to mate with your female (who happens to be their mother, but they don't know that). You need a much larger number of females if you want to successfully keep two drakes.