Aside from the insane heat on Daryl on the aftermarket, I assume that McFarlane is also doing this 2-pack due to the fact that these two are the stars of the recently released (and sub-par) Walking Dead video game.
Along with the Dixon Bros, McFarlane Toys is giving is a (low res and heavily water marked) look at the figures from Series 4.
Gas Mask Zombie
Riot Gear Zombie
Daryl (from Daryl/Merle 2-Pack)
Merle (from Daryl/Merle 2-Pack)
Look for them to start hitting toys stores and Game Stop some time in September.