The difference is that the unique gameplay elements of AC forced them to be inventive with their multiplayer, and it paid off. ME2 is a third person shooter. The RPG elements are not going to appear in any competitive multiplayer implementation, so you're going to get pretty stock-standard TPS multiplayer, I'd imagine.
Unless, of course, this is just co-op, which signs suggest it isn't (you wouldn't develop co-op at a separate studio). That would be a nightmare to implement as well, and would ruin all the continuity between the games. I hope Bioware proves me wrong, but DA2 isn't exactly the healthiest of signs.