Kill fast. Die faster. Abuse Immortality! Together...
Momentum is shared, it depends on you and your friend's inputs if it is preserved, boosted infinitely, or instantly erased. You can also animation-cancel all actions of your friend, and vice-versa, for insane techs.
The movement by itself is unironically as deep as your skill level. Hence, there is no in-game skill system, as all movement options are unlocked from the start.
Double Damnation's level design was tailored to its expressive movement, and so, every level is non-linear. For this reason, there are few levels, but they can never grow old, as you grow more skilled and traverse them in a different way - faster or safer.