In the end, I decided to just experiment and see what worked.
Specifying the Optane-accelerated Drive
I learned that Optane can indeed be paired with any installed drive in this system. Determining whether it can or can’t is simple. Here are (roughly) the steps:
If you later change your mind about which drive you want to accelerate, disable Optane again, reboot, launch Intel RST again, select the other drive, and enable.
Note: As RoHe specified, BIOS must be set to RAID (or Intel RST) in order for Optane to work. From what I’ve read on Intel’s site, it appears that only the Optane and HDD are paired in a RAID and other drives you install are not. Thus, whether BIOS shows RAID (as configured by Dell) or Intel RST (If you manually install the Optane software), the effect appears to be the same.
Boot Drive: Optane/HDD vs. SSD
Because I knew the orginal 1TB mechanical hard drive wouldn’t be enough storage for games, I added a 2 1/2” Samsung 860 EVO 1TB SATA SSD as the second drive. To make it the boot drive, I cloned the original drive using the free version of Macrium Reflex. Although Samsung provides free software for this task, it does not clone the Dell rescue partitions; Macrium Reflex can. In case things didn’t work out, I wanted to ensure that I’d have copies of them.
After installing Steam and transferring my game library from my old PC to the SSD, I eventually decided to put things back the way they were [Optane+HDD as the boot drive (C) and the Samsung SSD as (D)] by using Macrium Reflex to clone the SSD back to the HDD. Afterwards, in System > Storage settings, I set 😧 (SSD) as the default for program installations and the default everything else (documents, music, videos, photos, etc.) to C: (HDD).
Remember Those Recovery Partitions?
In attempting to restore the SSD to its orginal blank state, I discovered that initializing a disk does not remove any recovery partitions (there were several copied from the Dell drive during the initial cloning). Short of buying a utility, they can only be deleted with the built-in DOS diskpart utility. Instructions can be found in this article.