They don't really coexist though... not in this game. Absorb and Immune have precedence depending on the game port, and whether it is physical and magical. In FF5, FF6, and onwards, there is a distinction intentionally made. Just not this game.
i mean they can coexist in that a character will resist ElementX and be immune to ElementY. this is relevant should they be attacked with the two elements separately. that distinction wouldn't be possible with the hypothetical change i was responding to, as both'd simply be Immune.
Well, in the way it is currently set up for equipment, I guess the technical answer is yes. However, the Adamant Armor covers all the elementals that can be resisted naturally though armor (no armor resists Holy or Darkness, thus difficult to test this with.

Using FF4A US (GBA) to test with, apparently I could test it... but it's rather different.
There is buggy behavior that I'm using, but it's relevent... that some of the weapons are treated as armor, so the "buggy" Dragon Claw that is holy elemental would be treated as Holy resistance. Immune in the GBA port stacks (Yang was dealt 1 damage) when combined with the Adamant Armor. When I equipped the Hero Shield on Yang, Absorb overrode (or took priority over) the Immune bit.
anyway, i realize it's different when both elements are in one attack. i read about your Ice and Fire Claw example.. that's just weird. are the weapons not treated as separate strikes?
Physical attacks are treated radically different from magical attacks, as far as the game is concerned.
Here's the best example... the Dark Elf.
It is one of the few monsters that is weak vs an element (Holy) and absorbs the same element (Holy).
If you were to use the spell on the monster (which is not normally possible, but just for the sake of discussion), it would get healed from the Holy attack.
If you were to use a Holy elemental sword like the Legend Sword, you would deal twice as damage as you'd normally would.
I hope this explains the issue at hand.