Do you really think Chronopolis created a new timeline from 7600 (or 12000) BC onward? If it did, the 10,000 year past before Chronopolis doesn't need to exist anymore for Chronopolis to exist, because we're now on a completely different timeline where Chronopolis exists in 7600 (or 12000) BC, instead of being created in 2400 AD. Since we're on a different timeline, Chronopolis doesn't need to be pulled back to exist there.
However, I don't believe that's the case. Lavos pulled Chronopolis back to the time period where it existed. How could Lavos, who exists in timeline K1, pull Chronopolis to any timeline other than K1? I think, instead of the pull back creating K2 (as in nearly all other time travel circumstances), K1 remains, but is being overwritten by the new, Chronopolis influenced K1. Instead of having K1 and K2, you have K1 and K1a right on top of it. It could be theorized that K1a isn't even a timeline, just the actual K1 timeline's disguise.
Then, when Kid travels back 10 years to save Serge, we encounter a strange situation. Normally, the timeline from 1010AD forward would be discarded to the DBT, and a new timeline would commence. However, there are two timelines present there - one on top of the other. The original K1 is discarded at that point (as it should be), but K1a, the mask, if you will, remains. It's not a real timeline, so it isn't discarded, and it becomes Another World. The new timeline created by Kid's travel is Home World - thus the split in the dimensions.
Disclaimer: everything above is my theory only - none of it has been proven or validated
