I don't know why I didn't realise before, but all of the combine entities have been removed from Portal 2 (in accordance with Valve's recent trend of moving away from HL2 style games).
This means that much of DH is impossible to do in Portal 2's engine (unless there is another way of getting the Combine into Portal 2).
Oh well, my plan is to have Volume 2 be Portal 2 Exclusive anyway, so this way each engine has its own exclusive chunk of the mod, and the plot. (in which, much like Portal 2, the plot goes mental)
aww... oh well i have both games so im okay with it
Just use the Alien Swarm Engine. Portal 2 and Alien Swarm use the same engine.
Yeah, there is no mod support for Portal 2 yet, so you can't mount stuff like you can with the old engine.