It will not be in Crysis 2 however, I have high hopes for a modification to support it.
Mods tried if for Crysis/Crysis Warhead but it ultimately failed since the CryEngine 2 as a whole is missing the areas needed for coop to even be developed from (in other words the engine doesn't support it).
If the CryEngine 3 supports coop (I think its confirmed that it supports it), then a modification is highly likely.