Welcome to the forum Brick.
Some reading for you.
http://forum.honeyflow.com/t/adding-2nd-brood-box-under-the-super/7923
http://forum.honeyflow.com/t/add-on-brood-boxes/2827
http://forum.honeyflow.com/t/should-i-add-2nd-brood-box/9253
http://forum.honeyflow.com/t/adding-a-2nd-brood-box/9635
http://forum.honeyflow.com/t/how-to-add-a-second-brood-box/6961
http://forum.honeyflow.com/t/going-up-adding-a-second-brood-box/6628
http://forum.honeyflow.com/t/how-do-i-determine-if-i-need-a-2nd-brood-box/5258
http://forum.honeyflow.com/t/one-or-two-brood-boxes/9026
http://forum.honeyflow.com/t/do-i-need-a-second-brood-box/5645/8
Enjoy.