You don’t do that. Boot drive is separate. We suggest a small SSD or NVMe connected with a USB adapter if you don’t have enough drive ports. (64GB-128GB is fine)
You would be bettter off using the 2x 1TB NVMe as a pool for application or a fast data pool. I doubt you would need L2ARC, especial 2TB of it. RAM is faster than L2ARC and big L2ARC devices just eat into RAM that could be used for regular ARC. See attached links.
Boot Drive
TrueNAS Systems pool layout whitepaper
https://www.truenas.com/solution-guides/#TrueNAS-PDF-zfs-storage-pool-layout/1/