How can I pour a concrete floor/slab with two levels?
I'd pour the slab, leaving a 'hole' where the bath will be. Essentially, you will make a hole in the slab using forms. 

After the slab sets, remove the forms, come back and pour your slightly-lower slab in the hole that was left. 

Ideally, use some rebar in the original pour to tie in the new slab for the bath.

Alternatively, maybe consider pouring it all flat and use a transition strip between the concrete outside the bath and the slightly raised tile. 

Alternative option 2: if the whole house is going to be polished concrete, why not the bath too? Maybe stain the bath concrete a different color?