So today I was doing the bounties and the Daily Heroic story mission, and I had picked up all of the vanguard bounties. The whole time I was using my Auto Rifle, Sniper Rifle (Praedyth's Revenge), and Heavy Machine Gun. By the time I was done with the Daily Heroic story mission and the Heroic Sword of Crota, I had done a couple of the bounties. I also noticed that my Overcharge bounty was sitting at 1 out of 20. But I had not equipped my fusion rifle yet. I do recall, however, precision killing a Thrall with my Praedyth's Revenge sniper and watching the resulting explosion take out the one next to it. Perhaps this counted as a fusion rifle double kill? I'm not sure. The only other thing I can think of for me getting this is that during a public event on the Moon, a random person that was also doing the public event was using a fusion rifle. Perhaps I hit a couple enemies that this person double killed, and somehow it gave me credit? Either way, I did not personally kill 2 things with a single fusion rifle shot (yet), and I already have 1 out of 20 completed.