You can just watch it, even if plotwise its somewhat connected, but not integral. But I would DEFENITELY watch them in order just for the action itself (and less confusion, there's badass who's in both movies who play different characters, better see first to fully get the difference hehe), because the first movie is like nonstop all out one single building setting mayhem, while the 2nd one, is quite different, its more of a broader gangster story, even if it still has the insane action and MORE, its more widespread (like an hour longer then first almost) and slower paced fare. Mood wise I think its better to watch them in order to then compare them.