Why is that a thing?

Dogpool has real name Wade Wilson. I think headpool might too? Why oh why can this work?

I never read the comics, so I don't know the story.... but I'm guessing Deadpool got turned into a dog and a zombie head? So, their real names are Wade Wilson, so they are legal to use in Merge and Split.

Why is it allowed? Cause the rules say so.

So do Lady Deadpool and Kidpool, I believe.

Nope. Ladypool is named Wanda. The rest of them however are all Wade.

Alternate universes, actually.

Headpool is the Deadpool from the Marvel Zombies universe, who got decapitated at some point. But, y'know, lived on anyways because Zombie. I don't know what the universes are like for Dogpool, Kidpool, and Ladypool, because I've never read anything related to the Deadpool Corps, I've just picked this up from discussions and conversations I've overhead.

All the different pools are from different dimensions so they are all wade just from another universe that came together in the comics

Back when they announced the Split/Merge, WizKids stated that they wanted more player flexibility than strict Comic Accuracy. It's one of their better moves, I'd say.

You can get Dogpool/Bob, and other silly things like Two Buckys fusing into Cap and Bucky. It's just how the system works, and it's fun to see what you can get out of it in this case.

My favorite is still Granny Goodness and the new Mercy being able to merge in to the Goodness and Mercy duo from B&B

Let's not forget that they work with GSX Cable & Deadpool chase as well. That new Cable is pretty wicked.