You can carry two weapons and grenades at the same time, but not two heavy weapons. You can also take guns from enemy soldiers when you run out of ammo. The focus meter fills up as you fight, allowing Jack to use slow-motion and target enemies more easily. Almost every soldier can be taken out with a single head shot, which will put the game in slow-motion for a second or two.
There are missions where Shadow takes control, usually when you need to get keys or disable electric fences. As Shadow, he can go stealthy and sense guards, even through solid objects. He can take out enemies quietly and even pull them out of sight. However, if Jack dies, the game is over. If Shadow dies while you're not controlling him, Jack can revive him.