I was working through what the response from lsblk is telling me. Specifically, I see the physical (virtual) volumes sda and sdb, and I see the partitions sda1, sda2, and sdb1. What I don't see in Photon is the volume groups / logical volume (lvm) that I see from lsblk in Univention. Presumably no logical volume means no way to expand that logical volume across multiple partitions. So I'm going to take the relative silence from the community and my observations to mean that Photon simply doesn't support this. Unless anyone has any additional feedback?
I realize this is effectively a way to set up JBOD within an operating system, and is not something I'd ever want to do on a physical computer. But since all my systems run on RAID1 or RAID10 arrays, I get the benefit of JBOD without its inherent inability to tolerate a physical disk failure. And since I'm looking to use an appliance to replace a Windows VM with VMDK of over 3TB, the ability to expand my disk space by adding virtual disks is attractive because 1TB files are easier to manage than an ever-growing single VMDK.
Ultimately, I really like how slim and easy-to-manage Photon has been, but I'll have to weigh the benefits of the lightweight OS against its lack of a logical volume.