The best time to use it is any time that you want to reduce glare (thus increasing color saturation and detail) from non-metalic objects and/or darken a blue sky.
You won't get any results worth mentioning when it's overcast or when you shoot indoors.
The effect varies with the subject's angle to the sun, the maximum being 90 degrees. If you point your index finger at the sun with your thumb sticking up like you're pretending a gun, the direction that your thumb is pointing will be the direction of greatest effect.
I've heard from a few people that it is not a good idea to stack the two filters, but check with the maker of the polarizer to see if they think there is any compatibility issue. Generally though, the more glass you have stacked in front of your lens, the tougher it is to realize the quality potential of your lens. Just me? I'd use one or the other.