Two green screens in FCPX?
It depends, you can typically adjust a tolerance for the level of removal.  If the colors of green are closer than the next nearest color, you should be fine, if not, then you would have to do two keys, though I would expect you should be able to stack two layers to get the necessary transparency.  (Basically keying over a keyed image, since the result of a keyed image is a layer with alpha.)  

Keep in mind that lighting will also be a significant concern and you could always paint the floor tiles as well to get the same color on everything.