Does that meant that if you can't scale to get the 'two in one' casting you can't fix the wound? If so it would limit the spell by making you heal the entirety of the wound at once (thereby requiring more ranks) which many people seem to want.
Right, at least that's how I read it. I admit, it is interpretation that you can't recast the spell. To me, it makes sense (same with lore skills - if you don't know something, you can't just repeat it until it works. By the same logic, you could try until you fail, but no: The dice decide), but still: It isn't stated anywhere officially.
There are 2 concepts behind how one understands the spell:
a) I put magic into it, which does something good.
This would be repeatable. Some skills work like that, using the percentage column (e.g., pulling someone up, a tug-war, ...). Repeat until finished.
b) I try to heal a wound to the best of my abilities. The roll decides how successful I am (as with any skill roll, see above).
This isn't repeatable. If something is too difficult for a certain skill level, it can't be done. If a wound is too severe for your magical skill, you can't heal it. If a book is written too complicated you can't read it. If an item is too complex to attune to, you can't crack it. This is a fundamental structure in the whole game system, so I don't see why healing with a spell should work so differently.
By the way, Kasalin, your option makes the difference between Major and Minor Healing simply a headstart. I think this is unbalancing for the priest profession.