Redundancy/RAID on Airport Shared Disk?
According to this Apple support article:


  The AirPort Extreme (802.11n) and Time Capsule work with most disks
  that are not software RAID volumes (no more than one volume per
  physical disk). If the disk is a self-contained RAID that presents
  itself to a computer as a single volume requiring no software support,
  then it may be supported.


So it would appear that you need something that could be connected to the Airport Extreme that does its own RAID. Perhaps something like the Netgear ReadyNAS Duo, if you want to get a ready-made solution. You can also buy NAS enclosures with two or more bays, a couple of bare drives, and roll your own. Note that you would typically connect your NAS to the Airport via Ethernet (though some NAS enclosures allow you to connect via USB, which was your original plan), but since the limiting speed is the Airport wireless speed, I don't think that would be a problem.