That's the thing about Deadpool: he warps the reality of every medium he's translated to. And, in this print from Marco D'Alfonso, neither the Joes nor Deadpool has quite figured out how this crossover makes sense.
The details on this are really what makes it great — just look at the choking hazard warning up at the top!
Doing crossover art is a specialty of Marco D'Alfonso. On his website, tumblr, and deviantART page, you can see more. In a similar vein to the G.I. Joke (available as a print here) above, have Deadpool and Boba Fett, mercenary best friends.