Recently on a shoot I had combined my B+W UV thin mount 77mm filter with my recently acquired B+W CPL (also thin). Went great until I realized that somehow, these filters had become for all intents and purposes fused together. I cannot unscrew these things no matter what I do, and I can open a mean jar of pickles. So, FM friends, what do I do? Do you have a special way of dealing with this? Is this even something you've come across? These are expensive filters so I don't want to get too risky in what i try for fear of damaging, scuffing, scratching or otherwise warping them. Please help! Much appreciated.
If you don't have the time to buy them, then you can also try using two rubber bands.
There's a technique to removing filters--never grasp the filter too firmly. If you squeeze the filter too hard, you momentarily distort the ring just enough that the filter threads will lock. In other words, the resistance increases the harder you grip. And never screw the filter on tightly--only turn it until it just begins to stop and catch.
To remove a filter, grasp the ring lightly and evenly around the entire circumference, and turn gently. Another common mistake is to turn in the wrong direction, inadvertently tightening the filter. To avoid this mistake, the rule is simple: if you point the face of the filter toward you that is supposed to accept incoming light, turning the ring counterclockwise loosens the filter, and turning clockwise tightens it.
In many cases where the filter(s) is/are undamaged and have not suffered any impact, stuck filters aren't really stuck. They only feel stuck because you're exerting too much force and distorting the ring in the attempt to unscrew them.
As per what wickerprints says, this is a case where less pressure is more. If that doesn't work the rubberized pad that Monito suggested will work. The ones they use for opening large glass bottle lids. I have used them in the past. I leave the rubberized pad laying flat on a counter and turn the filter either on or off the lens keeping pressure pushing down, not too much.
I always carry several filter wrenches now, after having this happen on the road. They are cheap and lifesavers when double stacking.
I would have suggested a rubber pad (such as those for opening jars) but because one of the filters mentioned is a circular polarizer, using one will cause that filter to turn freely without inducing the necessary torque to unscrew them.
Surprised to hear two brass-ringed filters would get stuck to each other, but I guess it happens to the best of them. If you can't get them apart with a good pair of filter wrenches, perhaps consider sending them in to B+W and maybe they can separate them:
The rubber band trick helps. If you lay the non CPL on the rubber mat that should help too. It is helpful because of the even contact along the whole cirumference will resist deforming the circle as you twist. If there wasn't any impact damage, etc either of these should unstick 'em, and you don't have to go looking for a filter wrench.
Thanks everyone. I'll try some of these suggestions tonight when I get home. Trenchmonkey's right, didn't really need to stack them and it wasn't my intention to. Was out on a shoot and needed the CPL, and the UV stays on there 100% of the time simply to protect the front element. Wickerprints also makes a good point about the CPL which will simply rotate freely if pushed down on a rubberized surface. First method I'm going to try is the rubber band method since I don't have any filter wrenches. I was really careful when using them to not deliberately over-tighten, but somehow it must have happened anyway. I will definitely NOT be using this 2 filter combo again. It doesn't give me any optical advantage anyway, and I can just stick the UV in a filter pouch when not in use. Shows where hurrying gets you.
OK! So I finally got the chance to sit down to get this done... and, the rubber band trick worked straight away. Just had to be gentle and deliberate, not squeezing too hard and they came right apart. Whew. I definitely won't be doing that again.
Pointless combination...UV and Cir Pol. 
My faux pas was well before there was an internet 
