Getting pipes apart depends a lot on how they were put together. Many shops tighten the clamps enough to deform the pipe and that makes getting them apart without damaging the one you want to save impossible or at least extremely difficult.

Even if the pipes are not deformed many times they rust together and while penetrating fluid can work to your advantage it isn't always possible to separate two pipes. Sometimes you can use a die grinder to run a line through the outside pipe and apply penetrant along that line as well as inserting a large screwdriver and raise the outer pipe with a twisting motion. That will sometimes help release the hold of the outer pipe.

Anyway you look at it exhaust replacement can be nasty work. My experience is that when a muffler needs replaced it is worth the effort to try to save the pipes. However, if a pipe is bad chances are the other components is the system are on borrowed time.

Good luck
Randy

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Any twenty minute job is just a broken bolt away from a three day ordeal