I have just Comets in my tank and they like to chase eachother sometimes. As long as it doesn't come to fin-nipping, it is just their way of playing. When they are courting (getting ready to mate) there is a little chasing involved and some nosing. The male usually gently pushes the female with his nose and she will let him nudge her if she is willing to mate. If not she will just swim away. After a while he will get the hint. Because you have a common and a fancy, I would tend to believe that it isn't mating but just some playing, maybe a little competing for food. They LOVE to eat A LOT.