You mean probably the behaviour, when one of the degus tries to get to the other's back. And they are making arguish sounds probably too. Such behaviour doesn't have to be mating, but also telling the dominance and it happens almost everytime when you put new degus together in the cage. Dominance is important thing for degus and they must always make clear in the beginning who will be the boss, who will eat first etc. And they tell the dominance by behaviour similar to mating.

I can't tell you that you don't have a pair, because such mistake happens quite often mainly in pet shops, but this behaviour doesn't point to anything - 2 males, 2 females and also a pair can behave like that.