Ahh... yeah.. Erm... lemme think, It must be the same button, if you hold the button you use to make her pick things up, hold that button, where your health bar is, two rectangles should appear saying cover/attack push up or down to choose what she does. with online co-op four boxes appear with commands to command the other person to stop ect haha.
The button you use to call her over, hold that button lol plus on xbox if you click both sticks in at once it taunts the enemy to come to you, apparently anyway, its worth a try though, dunno if the ps3 still uses the stick buttons you should try though man.
hope I've made sence. tell me if you try it out.