To make a long story short, there's this virus called the Facade virus, and both Wade (Deadpool) and Nate (Cable) have been infected. The Facade virus is *supposed* to change everyone and make them blue, because the people using it (the One World Church) believe that making everyone the same color will somehow be the first step in stopping disagreement and fighting between people. Well, the virus *does* turn people blue, but unfortunately there's a pesky little side effect: it causes them to melt and die. (Ah, I love comics.) Since both Cable & Deadpool are not like ordinary people, it affects them a bit differently. In Deadpool, it still causes him to melt, but a lot more slowly because of his persistent regenerative powers. In Cable, it acts to block his telekinetic and telepathic abilities to control the techno-organic mesh that makes up part of his flesh, causing the T-O to start taking over his body and immobilizing him. Cable determines that the two have to swap DNA so they can pool their abilities in order to save themselves (yes, really).
And we get this:
(In case that's a little hard to follow, once Deadpool melted Cable absorbed him through his pores and then, yes, puked him out. While doing so he managed to use his powers to fix the glitches in the virus (which he'd figured out earlier on). He also managed to mix their DNA, which is Relevant To Our Interests, because it explains a fair amount of why they are now stuck together: in the story, Cable reconstructs an old space station of his which includes a teleport matrix. The way the matrix works is Cable says, "bodyslide" and he is teleported. Except that NOW, when he says "bodyslide," the matrix reads him and Deadpool as one person, and they are teleported TOGETHER, as one mixed up entity with 8 arms and legs. (Aieeee!) After a couple of times accidentally teleporting like that and having to tear themselves apart into 2 people again, Cable figures out that if they say "bodyslide by two" it will still teleport them both, but as separate people. The end result of this is that whenever one of them bodyslides, the other gets pulled along for the ride. Voila! Team-up explained!)
Oh, Nicieza. You so wacky. ♥