I'm not 100% positive about this, but I am fairly certain that this is a glitch that arises from Blizzard's shoddy patching of the Gmerc exploit. Prior to the server-side patch, mercs could have any number of item-based auras active simultaneously, and every time you unequipped/reequipped a Dream helm, it would create a new instance of the Holy Shock aura. Rather than fixing the problem (dream creating multiple instances of its aura), Blizzard just made a lazy patch and changed it so that equipping a merc with an aura item when he already has an aura would overwrite the first aura. Lazy, lazy, sloppy patching on Blizzard's part.
There's another, older bug, too- when your merc dies, he loses all item-based auras until you re-equip the items that grant them. This combines with the sloppy patch to create the glitch you're experiencing. When you first enter a game, your merc will benefit from all the item-auras he has equipped. When he dies, you have to unequip and re-equip all of the aura-granting items... but when you re-equip the second one, it overrides and disables the first aura (thanks to the lazy Gmerc patch). Long story short, if your merc has two aura-granting items equipped, and he dies, you will not under any circumstances be able to benefit from both auras until you leave and re-enter the game.
Mostly just speculation on my part, but it fits perfectly with the facts. Either way, no matter how this glitch came about (shoddy patching or something else), the only way to fix it is either not to equip more than one aura-granting item, not to let your merc die, or to simply leave and re-enter every time you resurrect your merc.
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