This one might be a title niche but I have quite a selection of neodymium magnets and these sometimes get stuck together. On the larger ones they can be a real challenge to separate. This has resolved the problem.

Support the magnets on something firm (I used plasticine and the floor) to keep your fingers away and then push the tool down between them. They'll spread apart but still stick to the sides. You can then slide them up to separate further and then slowly slide one off at a time.

Obviously be careful that you don't hurt yourself. You use this at your own risk.

This one will work up to about 25mm but you should be able to easily scale it up if needed.