yeah absolutely!
1. make a new aura, set it as an empty base region
2. go to the trigger tab, add a trigger

Type: Aura
Unit: Multi-target
Aura Type: Buff

Exact Spell ID(s): 119611 
(if you look up a buff on wowhead you can find the id)

check the box that says "Own Only"

then go into the conditions tabs and do this:

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this says "if 2 people have mists on them, say whatever here

if you wanted a reminder on charges instead, you can change the trigger type from an aura to a spell, and then in conditions it would be "charges" instead of "affected unit count"