Just a note, GM has Alpha Killer and Executioner on 16 as well as Relentless Predator and Eager Wolf. I've found that in a majority of my games as GM, by 16 I would have already picked up max stacks on Wizened Duelest. At max stacks, I can 100-0 nearly any hero and so the additional damage from Alpha Killer or Executioner seem a bit wasted at 16. So I normally pick Relentless predator on T6 because despite us having Archimedes with cleanse, I find that reduction in CC time is far more valuable since I'll be a priority target for CC once I dive in Worgen form. And as a final note on GM's T6 talents, Eager Wolf is a must pick for me on BoE because he can solo burn an immortal while the rest of the team runs interference.
Now as far as counters for double warrior, I can play Tychus fairly successfully given that situation. I've had a few games in HL where I had a flex last pick with dub war on the other team and had major success with Tychus.
For double support comps, those are run to enable high value assassins, GM and Illidan come to mind. Another situation for dub sup is for high sustained fights on maps like Sky Temple and Towers of Doom. In the former example, hard counter for the high value assassin and heavy lockdown in terms of CC chain would be the way for us to counter it. While for the latter example, heavy burst focused on a single target would be our priority, dub sup can heal what's dead.
Double siege is an interesting topic that may need another post since specialists have the most variance in their kits. A common trait of specialists that I see is their lack of escape, barring of course Sylv and Murky. So a heavy CC comp would work well against dub spec, or a comp that boasts a 2 man gank squad that can run interference on both specs.