Resistances not stacking?

[TwoBits]

OK, when I cast two battlefield-wide protection spells, say, Army of Gold or Serpent's Blessing with Gaia's Blessing, the various resistances don't add together (so still only 50% fire or poison resistance, for example).

Is this a bug or WAD? Presumably WAD, but if so, what's the reasoning for not allowing those to stack?

If the whole-battlefield spells don't stack, what other options are available for getting 100% resistance? Do the AOE spells still stack with the BE-type spells?

Thanks