I happen to have a cat skeleton model nearby (and two now-very-annoyed cats).
The tibia/fibula of a cat (another pic, items 13 and 16) have separation, much like humans. The difference is the muscle distribution. Here is a pic (and another) showing the rabbit/chicken-like muscle shape of the rear leg.
The rear leg of cats is hard to describe since the joints are off by one (they walk on toes, not heels). But once you locate the 'hock', your cat should be increasingly muscular above there. That's where the tib/fib are located. They have enough separation that you may be able to feel it near the hock.
If you are feeling below the hock, keep in mind you are feeling individual bones (tarsals), just like a human palm and foot have. You can see that on this cat skeleton diagram.