no they wont stack, one is an active aura and one is an on equip aura.
The highest level one will win, The only way they would stack is if the mercs aura was another skill. Ie. if the mercs aura was called "Merc shock aura" and had its own line in skills.txt and holy shock had a seperate line then they would tack. They dont though so the highest source of holy shock will win. (if you have around +5-6 skills on your merc then his own aura will be higher. at level 99 I think they have level 16 skills iirc (before + skills items)

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mercs, atleast the A2 mercs had 'merc freeze' so its very possible they have a new skill for their auras

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“Education has failed in a very serious way to convey the most important lesson science can teach: skepticism”

nah they were changed to the holy auras to grant the -20 enemy res since it wouldnt work on the merc freeze for some reason.

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hmm... so what merc should i get for my cold sorc? whats the BEST -res solution?

A2 merc with infinity.