Lumens measure light intensity, in other words it is how much force or energy they have behind them to penetrate material. So putting another bulb next to a light with a certain intensity does not amplify the intensity. You can kinda think of it in mph for cars. If you are driving 55 miles per hour and someone pulls up next to you going 55 miles per hour, you will not both then be going 110. You do however get benefit from extra lights because you can spread out the lumens and cover your surface area better. Even though using two cars won't get you to the location any faster they can each haul a certain amount of boxes so you still get benefit of using two cars over one. I hope this strange analogy helps a bit.