The author has updated her character biographies. The characters who are indicated as especially enjoying cloning are Hannah (see, eg, Episode 3), Rebecca (Episode 8 bonus) and Kelly (Episode 12). Tiffany enjoys "splitting," which you can see in her introductory story, Episode 8, "Extra Lives". (In the story, Tiffany becomes two different people, both with near-absolute power akin to hers but with no clue that they are both aspects of Tiffany or indeed that she has fabricated every aspect of their existence.) All the "omnipotent" characters can, of course, clone themselves. (See, for instance, Nika in Episode 8.)
Some stories have the reverse, where two characters become one, different person, with a distinct personality but the same powers. This has been seen in Episodes 10, 13 and 15. These "legendary girls" only enjoy a temporary existence, however.