When formatting an SD card for use in a SCSI2SD, you can assign multiple SCSI IDs to the card. Each ID represents a different section of the card. The downside is that other OSes will only look for the first device on the card, and don't know anything about the other sections; they just appear as unformatted space. The issue here is that it's hard to use another machine (like a modern workstation) to interact with the content of the SCSI card. Since the modern machine can't see the other drives on the SD card, it's difficult to transfer files off the SD card.
I wanted multiple partitions on the card that I could easily read from another machine. The solution was to tell the SCSI2SD to emulate a single, large disk instead of multiple smaller ones. Then I created multiple partitions on the single device. This isn't the same as having multiple SCSI IDs that can be independently toggled on the SCSI2SD, but it meets my needs.