Yes, the damage debuff definitely stacks. For more details see http://us.battle.net/d3/en/forum/topic/13349475072?page=1#7
Section that start with:
Proof for Multi-player Stacking of Debuffs
Special thanks to ButteryNugs for helping out with the multi-player tests.
10) In multi-player mode, each player can proc their own Toxin gem and see their own DoT damage. When they attack the mob, they stack up the 10% Debuff for subsequent hits/casts.
Video 5 - http://youtu.be/1e3Nt-uHYhg
Base Locust damage = 813
Single player punch-proc, then Locust = 894
Two player punch-proc, then Locust = 976
etc.
official DB broken,, our beta DB safe to use atm?