As we all know Lightning Dust and Rainbow Dash share a, well… a big rivalery xD So do their daughters Azura and Stormcloud.
 
The two Mares are interested in different things, but everytime someone gets more attention or has done something better than the other one, they get jealous.  
For example, Azura won another Volleyball game and even Stormy is not interested in Volleyball she tries everything to be the center of attention. So does Azura.
 
So why are they always hanging together and watch each other if they are enemies? Easily explained, because Azura and Stormcloud can’t show all the power they got if they are not pushed to their limits! And seen that way, they even need each other.
 
Maybe one day they will find out, that rivalery doesn’t mean, that you can’t be friends…