Go to any mission that has the explosive tip crossbow and then after you pick it up, go into scope mode. While still in scope mode, switch to your other weapon quickly and then tap the scope button on the touch screen repeatedly as fast as you can. If you did it fast enough, the sights for whatever weapon you are using should appear over the scope of the crossbow. Pressing the scope button again will make the sights go away, however you will still be zoomed in as if you had the cross bow scope. This does not increase your accuracy though and you can only go back to normal view if you die or leave the mission.