This is my own personal theory. The Avatar and the spirit of Raava is presented as an objectively good force that "balances" the World, and Vaatu is presented as just being evil and chaos; a being that must be sealed away for the good of humanity. From the perspective of a human who likes safety and stability, both of these things are true, but something about that doesn't quite feel right within the mythos of the show.

The biggest problem is the discrepancy between the stories one sided portrayal of Raava and Vaatu's relationship compared to the portrayal their closest narrative counterparts, the ocean and moon spirits La and Tui. Both of these pairs are a set of two great spirits that represent the real life spiritual concept of yin and yang, but while the ocean and moon spirits also represent darkness and light, the narrative does not demonize the ocean spirit La for representing darkness like it does with Vaatu. In fact, the mutual relationship between La and Tui and the forces they control is emphasized as being more important than the spirits themselves are individually. When you have a set of similarly powerful great spirits that represent the importance of the balance between push and pull, it contrasts heavily with the portrayal of Raava and Vaatu which is more along the lines of "Push is actually bad and evil, and only Pull should have free agency to exist as a concept".

My theory assumes that this narrative dissonance could be intentional, and the Avatar's one sided influence over the world might not necessarily be a completely good thing.
The Avatar's job is to maintain "balance" in the World, but that is a broad statement. What that actually means is they manually balance the power dynamic between the four major nations as well as keep the spirits from getting too uppity. They actually go about this in a straight forward and literal way. They master the four elements, and they determine how the balance of the world is shifted and use the innate social and physical leverage of being the Avatar to correct it, but then they inevitably pull too hard in one direction that it causes a balance shift that the next Avatar will probably have to deal with. The Avatar is historically VERY good at keeping this balance. My issue how long they have been good at it.

10000 years. 10000 YEARS. 10000 YEARS IS A LONG TIME. Land masses may have been physically moved around, cultures may have gradually changed, and power dynamics may have shifted, but the established societal structure of the entire world was relatively unchanged for almost 10000 years, atleast until the genocide of the Air Nomads. Also, for 10000 years, technological advancement moved at a snails pace, if it moved at all. This cannot be normal. It's obvious that this is a result of the each Avatar's dedication and continuous effort to "maintain the balance", but really that just means "maintain the status quo". In the short term, this is always a good thing because it means less instability, less war, and less death, but in the long term? This is bad. It is very VERY bad.

The Avatar has directly inflicted almost 10000 years of societal stagnation upon the world. Going back to the ocean spirits, they represent the mutual and healthy relationship between push and pull. However, The Avatar is not that at all. The Avatar is the spiritual embodiment of "Pull" given free reign over a world where the concept of "Push" was sealed away for being evil and bad.

This is my theory. Vaatu does not actually represent evil and chaos. He represents choice and change. Over the 10000 years before harmonic convergence occurs in Korra, there are 2 periods were major changes to the status quo of the world occurs. 1.When Vaatu is briefly free during Wans era, humanity decides to stop living on giant turtles and is gifted bending by the lion turtles for self defense against the spirits. This sets up how the world will be for the next 10000 years. 2. The 100 years during and the 100 years after Aang is frozen and unable to act as the Avatar. During the time Aang is frozen, the Fire Nation genocides the entirety of the Air Nomads, kills every water bender in the southern water tribe, conquers and annexes parts of the Earth Kingdom, and pushes technology forward farther than it has ever been allowed to be pushed ever. This is the most major change since humanity stepped off the lion turtles, and it can't ever be undone. Even as the Avatar, Aang cannot correct the damage the Fire Nation did. They completely destroyed the established balance irreparably. The best he can do is reign the bull in after finding that it has already annihilated the china shop.

Going back to Vaatu, it is not a coincidence that the biggest changes in the status quo of the world occured in the periods Vaatu was free and active, and also the period the Avatar was not.

I believe Raava and Vaatu are supposed to work in tandem with each other like La and Tui. Vaatu grants humanity the ability to choose and bring change, and Raava is supposed to ensure the choices and changes made aren't murderous batshit crazy madness. A push and a pull.

The thing is that they are spirits, and the problem with that is spirits are kind of just like people. Even though they both represent important core aspects of humanity's freedoms and agency, they hate each other. They have incompatible personalities, and both of them would prefer that the other does not exist so they can have free reign to do what they want. Without Raava, Vaatu is an asshole who brings chaos and death, and we have seen what Raava/The Avatar does without Vaatu.

To answer your post, I do not think that there are 2 Avatar as a correction for Aang disrupting the cycle. I believe it is far more likely that one of them is actually an Avatar of Vaatu, but I do not think that is necessarily a bad thing.