[@Pudding]
In which case you need a divine protection for more than one magic. One gate in the body means one magic (Unless you have a divine protection which says otherwise. Or use spiritual arts, but that's a completely different scenario altogether)
Doesn't matter if you're human, turtle, chicken or succubus. You need a divine protection to have more than one. (Also I get why in Avatar. Really Wind should have been able to use it to, but fire makes sense as well. Not as much sense as wind but eh)
The lightning in Avatar is more to do with friction, which creates heat energy. Fire = heat. Lightning is friction using wind from my understanding. So really lightning could be used by both.
But once again my point still stands that they're not stand-a-lone magics. Such as good old Emillia being classed as a 'water' mage despite using ice. (Or is she under fire? I forget. I just remember she shoots ice, and there was a discussion on the wiki not too long ago about fire using ice XD)
Also I suppose metal is generally found in the earth, so makes some sense once more. Just like crystals.