World to Cube projection Unity
You can let your perspective cameras do the work for you.
Set the fov of the floor camera so that it shows only as much as will fit on the screen.
Then, have the cameras at the same position.
Finally, have the floor camera rotated on the +x axis by half of the sum the fov of both cameras. For example, if the wall camera is fov 80º and the floor fov is 40º, set the floor camera to be rotated by 60º along the x axis.
This will guarantee that the view of the cameras do not overlap, and they will have the correct projection along their surfaces to create the desired illusion.