System Picker only does the job of blessing one chosen System Folder at a time. You may have several System Folders on one volume, but only one blessed at a time. So System Picker works fine, as long as you know into which System Folder to boot next time, each. A multi partition HDT formatted disk would allow to choose at boot time, though.

OS 9 comes with a Startvolume control panel that allows to choose from multiple System Folders on one volume. This might work with earlier versions of Mac OS, as well.