For some reason Rocksteady took out the silent dual takedowns seen in Arkham City (by name anyway), but they've been replaced with these ferocious 'Double Fear Takedowns' instead. Proximity-dependant, they're different to the more regular version where any placement of surrounding guards lets you hit the button and start busting heads - here you need two guards to be side-by-side, whereby the contextual prompt will change and you'll be able to do a special animation that decimates them in seconds. Make sure you've got an escape route planned though, as any guards within earshot will come running - and remember, you can use a double takedown to begin the regular Fear variety, essentially adding one more to your allowance when in the mode itself.