How do you kill someone other than your target in assassins creed brotherhood

Well in an online match, you have a primary target and secondary targets. To kill a secondary target follow the small blue arrows, and once you get close the blue arrow will grow larger and a "beep" sound will be played. Although this is more difficult to locate this target. But to kill another player who isn't your target is not possible unless they are a CPU. BUt once you kill one, you lose your target.