Same results here. System Picker can only choose between System Folders on one partition (it blesses one of them which is then used to boot).

It is actually possible to select partitions to boot from by setting the "bootable" bit of the partition. Can be done e.g. with FWB Hard Disk Toolkit (they call the bit "boot priority"). You'll have to unset it on all partitions with a System on it except the one you want to boot from. Clumsy.