Click on the existing ceiling/false ceiling to which you want to add another false ceiling, and go to the Create False Ceiling option in the lefthand panel.

You will then enter the mid-state, where you can see only the shape of that tiling created colored in grey, and the outline of its base wall.

Once there, click on the Add option in the upper ribbon, to add another shape. You will enter the Sketcher mode, wherein you may then sketch the desired shape. Click on Apply to save changes and return to the mid-state.

The false ceiling most recently added is the one that is shown over the others in the mid-state. To move the ceiling above/below the other, click on the ceiling shape and go to the Move Up/Down drop-down menu in the left panel. Then, click on either the Move Up or Move Down depending on whether you want to move the ceiling above or below another.

Bear in mind that the Move Up/Down tool only affects the visibility of the ceiling's two-dimensional shape against the others for ease of selection and editing, and does not affect its actual position in real-time.