Is the UV filter on the CPL or the other way around? I think if the UV is on the CPL then that'll be really tough to get off because the base just rotates. I currently have a UV filter on my 70-200 I want taken off but it's been stuck there for a while... so what can I say. Best bet is to try and get the threads warmed up enough so the expansion could help make them easier to screw off, but too warm and you might damage them, so just keep on trying~
And what brand filters were they? If they were cheap then I say abandon them or see what you can sell them for, although two filters together shouldn't be done unless you're combining multiple ND filters, at least I would assume the CPL is much nicer and the UV filter won't be needed, aside from the filter of mine stuck on the lens I don't use protective filters, I want my images sharp and a little dust on the glass does no harm