A Medley (半レ人, hanrejin?, lit. "Demi-Reyvateil"), is a term that was coined by Tyria as a partially serious joke to refer to Aoto's particular conditions. However, the term does have a more deep significance. A Medley is technically known inside the Archia Think Tank as an Active Fusion human, which is basically a human that has active Reyvateil genes, in contrast to the normal ones who have them suppressed, who together with the Third Generation Reyvateils are called Passive Fusion humans.
They are actually connected to the SH Server, but instead of the Soulspace Addresses that all the Reyvateils normally are assigned, this assigns to them Soulspace Addresses that the SH Server can't verify due to not existing within a valid range. This normally causes no problems, but if the person undergoes an emotional arousal that would be enough for a Reyvateil to craft a Song, the SH Server tries to reach the Tower of Life inside of that person's Cosmosphere. Since it can't reach it due to the Soulspace having an invalid address, the Cosmosphere is deleted and replaced with a large stream of garbage data, which ends turning the people into Ultimate Border Humans. This process is commonly known as the Border Disease, although common people are unaware of what is the true cause for it. Both the Medley condition, and the Border Disease are only found in the region of Sol Cluster.
The reason why the Reyvateil genes happen to be active on a large part of the human population of Sol Cluster is because of the preparations the Archia Think Tank made for the execution of their Human Evolution Project, which consisted in spreading among experimental subjects modified Reyvateil genes, so them and their offspring could be merged with the Antibodies in a much easier way once the project was finally carried out.
Finally, and as it could be inferred from the above, Medleys do have their own Cosmospheres, and it's possible to Dive into them. However, if any kind of grand wave interference happened within it, such a creating a new Song Magic, or provoking a Paradigm Shift, would cause the same outcome as an emotional arousal in the real world.