Got something in my mind as i read your Question..
How about making a spellbook, and adding two abilities..
the one with 60% to cast Heal and the other with 40% chance to cast complete Heal
now add those two abilities to the spellbook and give this spellbook to your Hero/Unit that shall get that spell.. now disable this spellbook via triggers.. The effect is, that the spells will still work, but you don't see the spellbook..
Finally add some ability, that doesn't do anything, just to show the player the ability and what it does.. should work =)