Neither does he.
Yes, you can use it to back up both, but (assuming that you're using TM) you'll need to partition it. There are 2 ways to accomplish the dual backup capability assuming that both computers are on the same wifi network.
1. If you have an Airport Extreme, hook up the partitioned drive to the Airport Extreme via USB and set up user account access to each respective backup partition for each computer in Airport Utility. Then set each computer to automatically connect to its respective partition on login under the User Login Items pref pane. Finally direct TM on each machine to use its particular partition as the backup drive. For your initial backup for each machine connect it directly to the AE router via ethernet cable or the initial backup will take FOREVER. Subsequent backups will happen automatically whenever you're logged into your home wifi network.
2. Do the same thing basically, but attach the drive to your iMac and set up user account access for the laptop on the iMac in Finder (this will only work with OSX 10.6 and later). Make sure the iMac is set to Wake on LAN Access. Also connect to the laptop to iMac for the first backup or it will take FOREVER. Subsequent backups will happen automatically whenever you're logged in to your home wifi network too.
With either method when you are away from your home wifi netowrk or when it is down, backups WILL NOT HAPPEN. You will also get a couple of warnings that the OS can't find the backup drive (server) whenever you restart and you're not on your home network.