The Boomshot achievement unlocked last night; was doing public, 1 v. 1, and only got a Boom kill in 1 of the rounds, so it looks like it counts then.
Will continue doing that and just score at least 1 Torque and 1 Hammer of Dawn kill in each match. I'll change it up to Private though instead as people are getting annoyed of being booted out of my public matches (there's never usually so many people playing!)
My concern later will be for Ranked, I had a little experiment with the user accounts, I could manage 2 vs. 3, if 3 vs. 3 then it seemed to have issues.
Once I mop up these other cheevos I'll give it another try, if still no joy, then I do have a backup plan, I can use my Bro's laptop and install the game disc to his PC for an extra player. I am using Daemon tools on my own computer.