They both are great characters but, witch came first because I've noticed when I looked at what comic they fist appeared in they're in two different comics but, you can see both of them on the covers:

Gallery

That wasn't Domino in New Mutants #98.  It was Copycat posing as Domino.  Tolliver had been holding Domino captive for over a year, during which time Copycat became a founding member of X-Force & initiated a sexual relationship with Cable.  The swap came to light in X-Force #16 (which is why it says "Domino II Revealed!" on the cover :) 
 
So Deadpool came first (publication wise).  But that was around the time that he & Copycat were still an on-again/off-again couple.  So when they fight in X-Force, it's  not just as two employees of Tolliver, but also as a sadistic break-up.

who can fist? Well either of them. But that is very dirty. How dare you bring such a dirty topic on this site!

Dynamic Domino?
The introductory tittle never sounds proper.