The first position listed is the first position they knew. So, if you train a player who is a DC to also play DMC, then it would show up DC/DMC. As far as I know, this doesn't mean they are more proficient at one versus the other.

Unless their statistics make them worse at one. For example, if you train a DC to also play DMC, but prior to this training had only improved his defensive and physical/mental chararcteristics, he is unlikely to be very effective in the attack (because his passing, shooting, etc. statistics will be poor).