You can always load the second round a slug.
http://www.theboxotruth.com/docs/bot3_2.htm
When To Use Birdshot
A friend of AR15.com sends this:
"I saw a gunshot victim, about 5' 10" and 200 lbs, taken to the operating room with a shotgun wound to the chest. He was shot at a range of six feet at a distance of just over the pectoralis muscle. He was sitting on his front porch and walked to the ambulance. We explored the chest after x-rays were taken. The ER doc had said 'buckshot' wound, but this was obviously not accurate.
It was # 6 shot. There was a crater in the skin over an inch in diameter. When the shot hit the level of the ribs, it spread out about five inches. There was ONE pellet that had passed between the ribs and entered the pericardium, but not damaged the heart at all. As you say, 'use birdshot for little birds.'"
It's worth noting that it may not have killed him, but it made him stop doing what got him shot and that is the objective of home defense.
It's also one of the reasons I said earlier that you made need to take head shots. A shot or two to the chest will slow him down at least and that is where I would start if possible. However, a head shot is more likely to knock his lights out in one way or another (dead, unconcious, blind, etc.). It's also worth noting that I keep spare shells on my shotgun with at least 2 shells of buckshot that I can transition to if necessary. Most importantly, train with your weapon and rounds; you need to be able to take accurate shots as quickly as possible. Keep shooting till he is no longer a threat (ie, running away, on the ground, dead).