my setup
I have the above setup with an Airport Extreme and a Seagate external HD. I occasionally have problems with one of my iMacs (on wireless) locating its Time Machine backup, but other than that, as I posted in another thread...
Time Machine and SuperDuper (bootable clone) for two iMacs and one MBP go to the Seagate 1.5TB drive, plus BackBlaze for remote file backups from the two iMacs.
I also use Chronosync to make sure that my work iMac and my MBP documents folders are identical.
This way, I have two backups of each of my three computers on the Seagate, my work files are doubled (being on both the MBP and the iMac), and if our house was broken into and everything was taken, including our Seagate drive, all of our pictures, movies, music and documents are remotely living with BackBlaze.