If you now have this:
| Medusa | V-Splitter | H-splitter | Rock |
| Medusa | V-Splitter | Rock | |
| Wall | Flag | Cat |
Then up followed by left will clear the first rock and give you a fresh cat. After that you can use the H-Splitter->V-Splitter to duplicate the cat.
I misremembered though - you only need one extra cat. I thought I had to replace the destroyed rock, but I didn't have to.