I think it would be possible if you use the DVD version of X-Plane, not the serial number version.
The DVDs hold both formats, PC and Mac. Just install the appropriate version, then make sure your DVD 1 is in the drive when starting X-Plane. You might even be able to run it on both simultaneously, just remove the DVD after starting on one, then move it to the other.
The total diskspace is fairly low, the version installed on my laptop with quite a few areas of scenery clocks in at under 20GB.