You could try to rearrange the decor, or possibly remove the 2nd JD are re-introduce him during the night after the lights have been off for a few hours. Jacks are known to be bullies (hence the name) what size are both? Male/male?
it could be possible that you purchased a female and their courting but most likely its two males establishing dominance if the fighting continues or gets worse over the next couple of days seperate them