A mate and I played through about two-thirds of Derelict in two player mode today. It's still very glitchy, but now playable to an extent. Just my thoughts on the experience so far.

• As previously noted, the tactical elements of the single player game have been taken out of co-operative to streamline the experience (The two player characters possess most of the abilities of the previous five man squad between them).  So far, this seems to work very well, except..
• With only two characters, the total amount of available firepower has dropped considerably, and made boss fights insanely difficult. The compromise to this situation was to leave other "Red shirt" AI marines in the level. They can't be directly controlled but can be made to follow either of the player characters.
• The co-operative mode marines come back to life a period of time after they've been killed (Except when both die at about the same time, the level restarts). Again this is to lessen the difficulty of having a smaller squad, also I think it's more important to keep the 'flow' of a game going in co-operative mode than to challenge the players.
• Except for a variety of glitches (Most annoyingly, some major slowdowns in certain levels) the game is pretty good in multiplayer. If it works out well, an online version may come in the future.