If they're two males you need to separate them, otherwise one will kill the other. Best to put one in the sump then sell it or return it and get one of the opposite sex.
First, the tank is too new and small to suppose two Mandarin let alone one. You need a good population of pods for them to eat, which your tank probably doesn't have. At this moment you need to check and see if you have two males, if so you need to remove one.