What I noticed while running Nexus last night was the following:
We were with 2 hunters in the group and on a boss I was paying attention to his buffs and noticed the following: double hunter's mark!
Now you can start yelling: just graphical, client side only.
I told my group and they did see it too and we tested it later on with our dmg meters, the AP does stack and we did more dmg with both on. Worked on the dragon on the end of Nexus too, not sure if it works somewhere else, didn't have time to test it trough on everything. I've searched for hunter's mark and hunter exploit and haven't found anything similar.
Enjoy and happy exploiting