For anyone who happens upon this same situation, my guess above turned out to be correct. But in addition to renumbering the scsi_id of each of the LUNs, I also had to renumber the controller_tid field so the controllers on the second node would show up with unique ids. Here's an example of the config stanza I used for each LUN:
<target iqn.2014-06.storage5.jonheese.local:datastore5>
# backing device for the storage LUN
backing-store /dev/drbd5
# SCSI identifier for the storage LUN
scsi_id IET 00050001
# SCSI identifier for the controller LUN
controller_tid 5
# iSCSI initiator IP address(ed) allowed to connect
initiator-address 192.168.24.0/24
</target>
So it was a tgtd configuration issue after all! Thanks everyone for asking the questions that led me to the correct answer!
Regards,
Jon Heese