SAS supports multiple HBAs however, so you could have 2 computers attached to an expander.
Right, but the expander unit itself is incapable of running on its own. Its like those ooooold school computers that had a secondary tower full of scsi drives, and a fat, four hundred dollar (they were expensive) scsi cable connecting it to the host computer. the storage tower isnt truly a computer / system, just a mass of harddrives, cooling, a PSU and either a controller, or a expander.