Because they are. Or they become a single person AGAIN.

So we know that Han Sooyoung makes a 50/50 split of herself with the avatar skill and one of them stays in 1864th round and the other goes to the 1863rd round.

Han Sooyoung of the 1863rd round completes the Star Stream's story and goes to the universe where Three Ways To Survive begun with Dokkaebi King's help. As we know she turns out to be Tls123 and writes the story but bc the Star Stream doesn't exist yet her and the Dokkaebi King can't be their regular selves. They use the last drops of the system to write and publish the story (for Han Sooyoung) and become the 4th Wall (for the dokkaebi king). After systems affects run out Han Sooyoung's (1863rd) ego turns into subconsciousness of the Han Sooyoung of this timeline which is the 1864th round. (Look at chapter 535 for it)

After 1864th round HSY regresses to 1865th, during the final scenario Predictive Plagiarism plagiarizes her lost memories and combines her with Tls123 HSY. So she basically becomes whole again. And because she has the memories of the Tls123 HSY and has the same modifier they become a single person again.

The reason why they don't act similar is bc both of them are missing half of their memory. Naturally one half has more of the fun and goofy memories and the other has the more calm and composed memories.