I have a Vanguard bounty ("Overcharge") to "kill 2 enemies at once with a Fusion Rifle, 20 times". I've tried cornering enemies to get the double kill, but it doesn't seem to count. Is there an area or technique that is most efficient for completing this bounty?
The temples under the Moon are a good place to earn this Bounty. Play your favorite Story mission or Strike and you will inevitably be attacked by swarms of Thralls who love to charge you head-on, in single file. They also have low health stats, so the fusion rifle is more than capable of tearing through them and getting three or four dead in one burst. I doubt you get additional credit for killing more than 2 enemies at once, though.
Pocket Infinite is a fusion rifle that fires like an automatic. It can make Fusion Rifle multikills much easier to acquire on any location.
In this video, it's described that The Last Array (Earth) is a pretty good place to get this bounty.
Hive Thralls are your best bet at nailing this one quickly.
Most strikes featuring hive will have tons of them, but I find the best ones to try (for quick repeatability) are actually either the fist of Crota mission, when omnigul spawns droves of them in, or the first story mission on the moon, once the hive gates open, there's plenty of them to massacre.
if you just want it done quickly, FR the Thralls, and let the Knights kill you. The kills still count towards your bounty, and you get put back to the last check point (the gates opening ) so you can do it again without having to re-do the whole mission from the start.