GPU advice

[Qebm]

Hello, this is the core of the PC I'm currently building:

CPU: Intel Core i7-6700K
Motherboard: Asus MAXIMUS VIII FORMULA
RAM: Corsair Dominator Platinum 32GB (2 x 16GB) DDR4-3000
Storage: Samsung 950 PRO 256GB, Samsung 840 Pro 256GB, (plus some mechanical HDD for "slow" storage)
Video Card: Asus GeForce GTX 980 Ti 6GB
Power Supply: SeaSonic 1050W 80+ Platinum
Monitor: Asus ROG SWIFT PG278Q 27.0" 144Hz Monitor

I already own all these parts (actually, I bought them months ago, always postponing the build, mainly because I'm going to make a custom watercooling loop with acrylic tubings ) and I also have a second 980 Ti.
My plan is to play most games on the single 27" 2560x1440 monitor, except for 5-boxing WoW (via IsBoxer), where I want to use a 24" 1920x1080 monitor for the slaves, keeping the master full-screen on the 27" (so I can keep the same exact UI when I play a single WoW).
Is the single GPU enough, or should I add the second one?
I'm ok with lowering the graphic quality when multiboxing (I reckon the cvars scripts for doing so on the fly when switching window still works in Legion?)

[Hor]

Personally, I'd go ahead and add the second card. You don't HAVE to have it for boxing, it will mean you can improve some settings while boxing, but that's not the main reason I would add it. I would add the second card because it'd be a sin to not put it in there with that monitor. For DOOM or Witcher III, etc, that setup will be a treat to play on, THAT is where you'll see the real benefit of the second card imo. Not saying it won't help boxing, but it's REALLY going to be noticeable when you're playing something from your Steam library.

[MiRai]

If you've got the other 980 Ti I'd say just use it. Put it under water to keep it nice and cool, and enjoy multiboxing at 2560x1440.

[Qebm]

Thanks for supporting my nerdy wishes! Love you, partners in crime!