No. Sharing a drive for multiple purposes is not supported with TrueNAS. TrueNAS is designed for the Enterprise Data Center market. As such dedicated drives is a useful feature.
Now it is possible to “share” a boot drive. But the general consensus is that if you have to ask the question, you likely won’t have the knowledge or skills to implement AND maintain boot device sharing without problems. We just don’t want you to loose data.
I have to waste an entire drive just for booting?
In theory yes. But, using virtualized storage for a virtualized TrueNAS installation is less reliable, and has on occasion failed. Again, if you have to ask the question, you may not have the knowledge or skills to recovery after a failure.
TrueNAS uses ZFS for RAID functions. ZFS supports Mirroring of 2 similarly sized drives, so any failure on one, won’t impact the data.
This is the whole point of TrueNAS, using ZFS for ALL RAID functions. ZFS is in a class of it’s own for data safety and reliability.