Normally, you shouldn’t mount a polarizing filter on top of an UV filter. If you do, there are big chances that they will be hard to unscrew afterwards, mainly because of the rotating part of the polarizer. You will probably end up removing both of them from the camera, just like below:
There are a few solutions to unlocking them out there, including the professional one of getting filter wrenches, but I’ve got a simpler method. All you need is a ribbon, like this:
Wind it around the filter so that it “grabs” both the fixed and the rotating part of the polarizer. Hold one end of the ribbon between the index and the middle finger and the other one between the index and the thumb.
Make sure you leave a little to have some slack here. Now, use the thumb to press on the little extra ribbon as this creates tension.
The tension is enough to firmly grab hold of the filter and allow you to use the other hand to unscrew the UV filter.
The good thing about this method is the ease of finding a ribbon. You can use one from flower bouquets or even a shoelace! It is usable just as well to unlock a filter that it is stuck on the camera.