When these types of couplers become stuck, they can be difficult to remove (as you have discovered).
What usually works is the following:
(1) Spray a generous amount of a good penetrating oil such as aerokroil, PB blaster, or WD40 into the coupling to saturate it.
(2) Then, use a heat gun (not a torch or other high heat source that could damage the metal) and heat the coupling to expand the outer shell. Spray more lubricant into the coupling.
(3) Twist the lines in opposite directions to break loose the frozen metal
(4) Push the coupling tighter together as much as possible; this allows room for the garter spring to expand freely when the release tool is inserted
(5) Now, inset the release tool and make sure it is fully seated. Then, holding the tool tightly in position separate teh connection using a twisting motion as you pull.
This works in almost all cases, with a little patience and a lot of penetrating oil
There really is no other secret way to get these couplers apart;
If it is corroded too tightly to be able to twist it loose, then the only other option would be to cut the outer part off using a dremel or some similiar tool, and replace the line as part of the job.