I wonder... is this more common in all animals that have average litter size >= 2? Or is there something else special to cats that explains this phenomenon?

Half and half chimera is just the more unique variant, iirc, at least for humans. The more common type would just look splotchy if the different parts even happen to color differently. The patterns usually follow Blaschko's lines but don't have to.

There are also more basic forms where people will just have certain body parts with different DNA, like an extra blood type or other less consequential things.

Whoa, that's crazy. It's basically like having two cats in one!