WARNING: SPOILERS for The Flash season 7, episode 4, "Central City Strong".
The Flash season 7 dropped the biggest plot twist for Killer Frost yet as Caitlin Snow's status quo in the Arrowverse gets a shake-up. One of the characters that have been part of The Flash since the beginning is Caitlin, who has had a big journey over the course of seven seasons. While she wasn't a metahuman before Flashpoint, after Barry reset the timeline, Caitlin suddenly had ice powers as well as a second entity living inside of her - Killer Frost. While Killer Frost had her villainous arc in The Flash season 3, the later seasons saw her trying to reform alongside making the situation work with Caitlin, who came to terms with her powers. The Flash season 6 is when Caitlin took a backseat, while Killer Frost was more featured than ever to flesh out her persona as well as her dynamic with the other characters.
However, the latest Killer Frost twist is perhaps the biggest one The Flash has pulled. In the episode "Central City Strong", Caitlin was suffering from headaches, specifically after the final battle against Eva McCulloch a.k.a. Mirror Master (or Mirror Monarch). Whenever Frost was out, she was having the same pain and asked Cisco Ramon to run some tests on her. But Cisco didn't find any answers after giving her a check-up, leaving Caitlin and Frost without answers. However, for the episode's post-credits scene, Cisco (along with viewers too) is given a huge surprise when Caitlin shows up to show him something. While the headaches were fortunately gone, Caitlin revealed that she and Killer Frost were no longer sharing the same body as the latter pops into the room next to her.
The Flash doesn't explain instantly how this was even possible, but it still changes everything for Danielle Panabaker's Arrowverse arc going forward. Because this took place only a week after they defeated Eva, that showdown must have triggered something in Caitlin and Frost's meta-connection. Even though they don't initially have any answers for Cisco, that doesn't mean they didn't make this happen intentionally. Supergirl season 3 introduced a version of Black Kryptonite, called Harun-El, that allowed a Kryptonian to be split into two separate beings. Thanks to Crisis on Infinite Earths, they all live on the same Earth now and it wouldn't be out of character if Caitlin reached out to Lena Luthor for help.
Even though Harun-El would only work on Kryptonians, someone as gifted as Lena could easily have modified it to work on a metahuman. In this post-Crisis Arrowverse world, it may have evolved to the point where it can separate not just Kryptonians. That would be the most logical answer if this was something Caitlin and Frost did on purpose. But given the complexity of metahuman genetics, it could be that this was always inevitable as they got older. Assuming this is permanent, this would be a way for the two characters to interact physically with one another, as opposed to one talking to the other while Panabaker provides voice-over for the other half.
There's the chance that this was done to possibly send one of them off after this season, because having Panabaker play two characters every episode would increase her workload. It also wouldn't be surprising if The Flash merges them together as one down the line, depending on what answers they and Team Flash find. In a lot of genre stories like this, there's sometimes the twist of one not being able to live if the other dies. In other instances, whenever a split like this happens, there can sometimes be side-effects, such as two personas feeling sick or weak unless they are close together or bonded as one. However this goes, The Flash has officially kicked off the next big story in Caitlin Snow and Killer Frost's Arrowverse journey.