I only found one closed thread (Installing on NUC8i7HNK with 2 SSD) on how to add that second SSD to a system (exact specs as dappnode extreme), and I thought I’d add here what I found to be a better tutorial on how to add an SSD and extend to get a single 4TB logical volume.
I followed the commands in the post above to the letter with my physical and logical names and it worked beautifully. I hope it helps someone else!