We are training our two cats on two toilets and have had to step back
a couple of times due to butt splash
. We took the advice found on this forum to lower the water level, which only worked for one toilet. That was by removing the small fill tube from the pipe that fills the toilet bowl. Let that tube just drain into the tank, rather than into the pipe. You might want to secure the tube to the outside of the pipe with a plastic twist tie or a piece of string. That step lowered the water level sufficiently on one of our toilets, but not the other. So on the second toilet, we also weighted down the flap at the bottom of the tank that opens and closes to fill the toilet. We secured a large hardware nut and a few smaller ones to the last metal ring on the chain until the flap was weighted enough to close in a shorter time, allowing less water into the bowl upon flushing. We used plastic twist ties and some metal thread to secure the nuts to the chain. You'll have to experiment with how much weight; too much, and the flap won't open and the toilet won't flush. We still get a good flush, but we’re no plumbers and offer no guarantees.
Good Luck, and wish us luck!