For a campaign setting I am working on I have a set of outsiders that are tied to each other, One good, one evil. They know what the other is doing instinctively, but are unable to harm each other directly. They need to be about CR 7. being mirrors of each other is a plus, and its best if they are not only good and evil, but glow tremendously or exude magical darkness. and have light and dark themed abilities

Is there a set of monsters that has anything like this already?

if not, can some one homebrew one? I've got a few ideas myself, but I'd like to hear your take on the subject.