I'm super bored so I wanted to point out that there are some abilities which have a unique force multiplier effect when doubled with themselves. Snipe is a good example, but ground Spikes is probably the best one.
Let's say we have a level 10 Sedna with just Nimoth's boosting her armor. She'll be at 1328 armor, which is 34% mitigation, aka she takes 66% damage from autoattacks.
If we hit her with one rank 4 Ground Spikes she'll drop to -172 armor, which is ~-7.4% mitigation, aka a 62% damage taken increase. Not bad.
But if we hit her with a second one we can drop her armor down to -1672 armor, which is -201% mitigation, a full 300% damage taken increase (from autoattacks).
So there's one example of a legit force multiplier from DG stacking... and yet how many people here really care about double QoT? It's preposterous and it shows that people don't really care about balance when it comes to doubles, they care about being spammed with abilities they find annoying.
End of Obscenitor's quote
QoT is the least-played DG. I didn't even know till this post that it has armor reduction. Reg and UB are the most-played. See where the discussion goes? There surely other good combos.
UB: spit (and for some reason no one's mentioned how badly a multi spit can wreck towers, allowing easy map control), grab, slowing (there's only a couple items/skills with which you can hope to get out of their range once they've started hitting you)
Reg: range (wreck towers pretty well without taking ANY damage), snipe, mines
Sedna: Pounce Pounce Heal Heal Pounce Pounce Heal Heal walk out of your base over your dead body. I see this one alot. If they get in range of you, whether or not you have a partner in the lane with you, you're gonna die if they chose the right items
Rook: 2x blink, Boulder Roll, 2x Hammer Slam, 2nd Boulder Roll and slap if needed, towers all over the place to teleport anywhere needed (I haven't seen this, but I think it would be hilarious to gank people like that)
People keep saying double Regs suck, but I think they've been playing against some of the many noobs that don't even watch the DGs in the other lanes to snipe them. Most of the Regs aren't very good, and it's usually GG if one of them is bad.