Separate them ASAP.
In current situation, your dat may kill Jar or let Jar grow >8" and it will kill dat. You need to sacrifice one of them if you plan to keep them together.
what if there really not biting eachother but just trying to show whos boss, the dat would get close to the jar and the jar would just sorda chase him away. i put a tree decor in there and it seem that its working a lil bit. and i did do research they said a jar and datnoid good tank mate espeacially when same size as juveniles.