The Battle Ring should be big enough for the two enemies to move inside.

Some mini bosses and all the bosses should be able to pass the ring. Actually if you are higher level and the Battle Ring skill is not big level you should be also able to pass it. That's one more reason for you to add some skill points to this skill. Battle Ring cannot be combined with Doubles. When you have summoned doubles and you cast Battle Ring, only the original character performs the skill.