I have installed line voltage floor heat in a shower, I forget the brand though. You need to make sure it's specifically listed for use in a shower area. I made sure I had all my ducks in a row for when the inspector called, and he did. He didn't really like it, but it was good to go.
I also think its a dumb dumb idea and told the homeowner the same thing, that the water heats the floor. They still wanted it, so I put it in.
You can't connect two floor kits in the floor. You need to run both sets of leads up into the thermostat and splice them together there.