As mentioned by @Captain_Morgan (while I was digging up the JIRA ticket) TrueNAS doesn’t support multi-actuator drives at this point.
There’s a Suggestion/Feature Request in the “gathering interest” stage right now, that you can sign in and add your interest (with the thumbs-up) and logs (by using sg_vpd -p sn /dev/sdX and sg_vpd -p di /dev/sdX for two LUNs that are part of the same physical drive) to:
https://ixsystems.atlassian.net/browse/NAS-126776
It is possible to create a pool out of them but as mentioned you’re pretty deep into the Here Be Dragons waters with this as you risk having unexpected failure/replacement cases with two logical disks on one physical spindle.