Since it's not a popularly used thing, there may not be a right word for it yet.
But there are some examples available on Google (and Google Books) for "reciprocal followers", and I'd second what John Feltz suggested.
Reciprocal -- Vocabulary
Reciprocal describes something that's the same on both sides. If you and your sister are in a big fight on a long car trip, you might resolve it through a reciprocal agreement that you'll stop poking her and she'll stop reading road signs out loud.
The word mutual is a near synonym in most uses: reciprocal/mutual friendship, describing, a relationship in which two people feel the same way about each other, or do or give similar things to each other. If you tell someone you like them and they say, "The feelings are reciprocal," that means they like you too. In math, a reciprocal is a number that when multiplied by a given number gives one as a product.
I'd also suggest "reciprocated follower", which, again, has some usage examples on Google.
Reciprocate -- Vocabulary
You reciprocate when you return a favor, return a compliment, or respond "the same to you" to the angry guy in the car you just passed. In short, you react to an action, statement, or emotion by mirroring it.
This one comes from the Latin verb reciprocare, meaning to move back and forth. Its root makes it sound as if reciprocate implies only a physical action, but it's also used for the less tangible. If you fail to pay the rent, your landlord might reciprocate by evicting you. If your crush acts like he doesn't know you exist when you pass him in the halls, it's probably safe to say he does not reciprocate your amorous feelings.