Buff/debuff stacking and 4 man barb xp build

[NetQvist]

I multibox four barbarians and lately I've been wondering about debuffs and buffs stacking, seems like it's pretty random. For example there's reports that all but one rune on WD Piranha stacking between multiple players.

The team I'm considering running is two dps with cubed crown and ruby for 82% xp and two zdps with 227% xp

The skills:
DPS Barbs:
Furious Charge - Merciless Assault
Whirlwind - Blood funnel
Sprint - Forced March (Can be switched to WotB and the cube zodiac, however I dislike the movement speed difference to my zdps whenever the sprint timing is incorrect)
Threatening Shout - Falter (Might do one taunt) ?STACK?
War Cry - "Veteran's Warning"/Impunity ?STACK?
Battle Shout - Bloodshed

ZDPS Barbs:
Furious Charge - Merciless Assault
Whirlwind - Blood funnel
Sprint - Forced March
Threatening Shout - Falter ?STACK?
Ignore Pain - Mob Rule
Call of the Ancients - Duty to the Clan ?STACK?

Gear:
DPS Barbs:
Cubes: Furnace, Crown, Elements/Zodiac
6 Wrath, 2 BK
Chilianik's Chain
Strongarm Bracers ?STACK?
Focus, Restraint
Hellfire Amulet
Trapped/PainEnhancer, Stricken, Taeguk

ZDPS Barbs:
Cubes: Vigilance, Crown, Grandeur
2 IK, 2 BK, 3 Borns, 3 Cain
Hellfire Ring, Amulet
Leoric's signet
Pride of Cassius 6 seconds
Strongarm Bracer's ?STACK?
Gem of Efficacious Toxin ?STACK?
Defense Gems

To sum up the things that I wonder if stack:
Threatening Shout - Falter, no clue about this, hoping it does stack to 100% additive though (Should be easy to test if I ever get around to it)
Call of the Ancients - Duty to the Clan, harder to test and even if it doesn't stack it really doesn't matter
War Cry - "Veteran's Warning"/Impunity, Easy to test but once again never getting around it...
Strongarm Bracers, Unsure but I think they do
Gem of Efficacious Toxin, Should be stacking

Well that's what I'm wondering about right now, any improvements to the build would also be welcome.