Yes, they stack.
The only thing to keep in mind with -Damage is that, it's sorta tricksy, and is resisted by a target's own Damage Resistance. So if you hit a character who has 50% Smashing Resistance and 0% Fire Resistance with a -70% Damage debuff, the target's Smashing Damage will be reduced -35%, and Fire Damage reduced -70%.
Down to a minimum of 10% daamge.
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Thank you. 
Apparently, I play "City of Shakespeare"
*Arc #95278-Gathering the Four Winds -3 step arc; challenging - 5 Ratings/3 Stars (still working out the kinks)
*Arc #177826-Lights, Camera, Scream! - 3 step arc, camp horror; try out in 1st person POV - 35 Ratings/4 Stars
...from different powers?
For example: My Corr /Kin uses Void Judgement (-50%), then fires off a Fulcrum Shift (-20%). Do the enemies that get hit by both deal 70% less damage? Or does the higher of the two values overwrite the lower?
Apparently, I play "City of Shakespeare"
*Arc #95278-Gathering the Four Winds -3 step arc; challenging - 5 Ratings/3 Stars (still working out the kinks)
*Arc #177826-Lights, Camera, Scream! - 3 step arc, camp horror; try out in 1st person POV - 35 Ratings/4 Stars