Question:
It is strongly recommended that i use two scsi disks in a external case -
can i use only one with two or more partitions on it for the "shared
device"?
Answer:
I am pretty sure that you have to use two physical drives. If you are using an external RAID, you can create two logical drives called LUN's that can be managed as if they were two physical drives. But on standard SCSI disks this won't work.

Leon