It's not possible to tell in advance, if some degus will fight. You just have to try. Every degu has a different personality, some degus like each other and some don't. It's the same as with people.

Statistics say, that two females or a pair fight less often than two males. But fighting females aren't rare too. On the other side degus are sociable creatures and in most of the cages they are glad to be together and sad to be alone, so it's really worth a try.

To minimize argues and fights in the beginning, it's better if new degu is as young as possible. If he is just a baby, the other degu usually takes it as parent his baby and baby also couldn't fight much yet, so they get used to each other more easily.

What to do if they can't live together? If you have the chance and space, you can let them in two cages and buy each of them another friend, so that they are not alone. Or you can think of it from the beginning and talk about it with breeder, so that he can take the new degu back or exchange it for another. Often it helps to get new degu from another breeder, because all degus from one breeder smell similar and there is a bigger chance that your degu won't like any of them.