Its both their dots and their WS that eat our stacks. As long as both slingers have same gear, hybrid gain form my stacks exactly the amount I would gain, so raid-wide dps is the same. This is however only true until last 30%, where eating stacks leads to decrease of dps, as DF cant fully utilize +15% buff from skill tree
Good point about the sub-30 ability meaning hybrid (or ss) won't mix as well as another DF.
As to dots vs WS eating it, I just meant that as dirty fighting you make sure to have WS eat HB stacks, and the others wouldn't be timing their WS to line up with your HB (or wouldn't have them at all, and with less vital shot damage to boot, if you're raiding with sharpshooters/scrappers).
Thinking more, though, it's not so bad and could be quite good: you only get to eat HB stacks with the first WS in rotation, and if anyone else is casting WS then too, you could get more raid-wide dps out of HB actually by having WS's consume a greater share of the stacks.
In particular if you have two DF gunslingers, you ideally want them to have their rotations "out of phase" (i.e. one uses HB and then you both WS, and then after a few gcds the other uses HB and you both WS) and they'd each do more dps than they would alone. I guess you could test this on a dummy with two people hitting it. Or maybe you don't need them to be out of phase to gain this benefit: what happens if there are two HB's up on the boss at the same time?