It doesn't matter which is on top. However, before adding two filters of any kind, consider the fact that any glass hung on the front of a lens will degrade acuity at least a little. Multiple filters can also cause vignetting. See below.
Regarding the vignetting question asked earlier, the reason that some polarizers cause vignetting with ultra-wide lenses is that the filter barrel is usually taller than a standard filter due to the need to have both moving and stationary elements, which can impinge on the angle of view. There are some filters designed for wide-angle use that have slimmer barrels. Something else to consider is that a polarizer is difficult or impossible to use with a lens hood in place, so you'll often have to remove the hood, adjust the filter, then reinstall the hood if you're concerned about strong light striking the lens and causing fog or flare.
The closest I can come to the specific question you asked was to put a polarizer on my Nikon 12-24mm DX lens, which takes a 77mm filter. No vignetting even at 12mm, but that's not to say it wouldn't happen at 10mm. Also, if I add another filter to the polarizer, I get big-time vignetting at short focal lengths.