how do you get 2 pieces of plastic tube unstuck?
Are they PVC? Is there a joint or fitting over them (a bigger piece of PVC that both tubes fit into)? Did someone else assemble it (or did you assemble it with a PVC glue)? If you answered yes to all three of these, you are in trouble. PVC is often assembled with glues which can be very difficult or impossible to undo. It is often easier to just cut the PVC and then extend it with a new joint.
Try this, make one end cold and the other end hot. The cold pipe will contract and the warm pipe will expand, making it easier to separate.
