Well no, sometimes maybe.
Use it at the opener as usual. Save the other DS for when you have some sweet procs/buffs and such. Or when the add/boss takes more damage for some encounter specific reason.
It's quite nice to have when you and your guild really needs that extra damage too.
Just popping it without any procs or reasons seems silly in most if not all cases.
It's impossible to stack the two DS, no matter how much you press it, you can only use the second charge after the first DS runs out.
Taking that into account and the fact that Imp Swarm's cooldown is variable, what I usually do is use one charge of DS in the opener along with Imp Swarm and the Synapse Springs engineering CD. Imp swarm's CD being about 1min30s, and Springs being 1min, I can make sure I can always stack all 3 CDs every 1min30ish instead of every 2min without the talent.
All in all, most in-raid situations will require either Archimonde's Darkness or Kil'Jaiden's Cunning. I see Mannoroth's Fury as a more trash-oriented talent.
Aha. I thought I read off one of the spec threads that the buff does stack. Thanks, guys. It's no a no brain-er at this point. I here, for Demo at least, Mannoroth's Fury on the right fight out shines Archimonde's Darkness by a fair amount.