ummm... one problem. A story is 10 ft. up so two stories is 20 ft. high, jumping twenty feet down to solid earth would kill the normal human being even if a roll was performed. I hope your not relating a story to the kind of halo story, you fall 40 ft. and your shield is damaged. :shakehead:
http://www.youtube.com/watch?v=sb7C20stK78&feature=related Sorry. But he didn't even roll, and came out walking.
EDIT: Back to the roll idea... They could do what Mirror's Edge did.
Dropping from extreme height (approximately 2 stories) you can do one of three things:
1. Do nothing, hard landing, take full damage from the fall, and losing momentum.
2. Hold the crouch button as you're coming in for the landing, reducing damage slightly, while still allowing you to shoot.
3. Tap crouch in the moment of impact, and go into a roll. Negates almost all fall damage, BUT leaves you vulnerable and unable to fight back.
EDIT: http://www.youtube.com/watch?v=WEeqHj3Nj2c&feature=feedrec_grec_index