The Quicksilver Reflex Booster gives player additional option to engage two humans at once in a single takedown, lethal or non-lethal — instead of only one, which is available by default.
To initiate a double takedown you need to be near two humans standing close enough together — normally in a 2 meters radius either from Jensen or from one another. It means that you can still perform a double takedown in a position where you're 2 meters away from one enemy and, say, 4 meters away from second enemy — if enemy #2 is close enough to enemy #1. This will result in visually noticeable "teleporting", where Jensen and both enemies are moved from their original locations into fixed positions for the cutscene.
Also note that always the two closest humans are targeted, which can be a nuisance if civilians get close to enemies by mistake, leaving one of the intended hostile targets unharmed.
If you have "Show Cover/Takedown Prompts" option enabled, it will tell when you can do so by showing a special double take-down symbol on the bottom of the screen.
Energy cost is the same as for single takedown — 1 energy bar. Noises produced by double takedowns also remain at the same levels: approximately 1 meter for non-lethal and 25 meters for lethal.
For the discussion of double takedown tactics and experience bonuses, see the Takedown page.