Try using some basic hand tools to get it apart, but be careful, as it may dent the material.

I don't know what your pipe is made out of, but I use to have a metal one where the connecting pieces would always get stuck together.

Try using a pair of pliars (sp?) to get it apart.

Maybe try putting it in a vice, and using the pair of the pliars to get it apart.

I found the best way to get those pieces apart was to heat it up really good with a lighter, then use a pair of pliars on one end to unscrew it, or the vice.