Males and females of various species may share nest duties. Cockatiels are well known for this, and males typically take on one shift where-as the hens take the other shift. IRN's may also have shadow rings, which are not true rings in themselves.
Two females and two males may act like true pairs, even going as far as mating and feeding each other, but never being able to fully reproduce!
About 5-7 days after incubation has begun, you should be able to candle the eggs (using a flash light) and see if there are any red veins. If they are male and female and have successfully copulated and fertilized the eggs, you'll know for sure! If they haven't, either they are too young and inexperienced or you have two females.
Since you bought them "as a pair" - is there any chance that the two could be related?