I came up with another way to separate a stuck mast.
I got 4 hose clampls from Bunnings: two 33-57mm to fit around the mast and two 17-32mm.
I've put the smaller clamps around the side of each of the larger ones.
Then put the large clamps on the mast to each side of the joint point. I left enough space between the clamps because I didn't want to clamp down on the ferrule. I used rubber mats as a padding between the clamps and the mast, and then tightened the clamps as much as I could.
Then I stuck the car jack between the clamps and slowly started jacking up. The smaller clamps' rings provide support for the jack.
After a while I heard a cracking noise but no effect. I released the jack, loosened the clamps and rotated them on the other side of the mast. Then I jacked up again.
After a few minutes of that the mast finally started to come a part, and I finally had it in 2 pieces again!