Stacks. Infact stacks with everything in the game. Always has. You can even stack "protection from chaos" - from a planar gird with this. So if the caster is chaotic evil, you get +4 to saves, stacking with all.
Infact most spells that boost saves will stack with items of the same bonus type. I think the game just treats spells and items as seperate bonus types, probably by design to make those buffs more useful to our loot-gauged characters.
Holy Aura is another example seemingly type as resistance too, yet stacks with resistance items.
Apparently this is some kind of thing that most players don't know tho.. Always get weird questions when I ask for these buffs.
Really good buff every divine/arcane should keep loaded given how easy it is to fit in and its low sp cost.
Interestingly, for I think mod0-2, this spell actaully made you 100% immune to all crowd control spells in the game. Was not known by most players that it did that back then either.. But I certainly knew heh, made some quests incredibly easy.
Currently it only fuctions as dominate immunity.