[Twan]

All spell buffs of the saMe type use non cumulative effects (you see when you mod spell that they just trigger the same effect, coded not to stack, with different aoe, range, etc or eventually several different effects for one spell).

All +MR spells (iron will, antimagic, resist magic, army of lead) use the same.

eagle eyes/aim/wind guide = same effect.

resist against x = same effect (army of gold + fire ward aren't cumulative ; nor resist cold + breath of winter).

etc...

There is no exception I know, but spells are cumulative with items effects.