Apple's Bootcamp instructions make no mention of how to proceed with a dual-drive iMac. Are there any differences in formatting/installation procedures involving either or both of the drives? I am new to Macs and don't want to muck up what is working well.
If you have an internal SSD and HDD I think it comes down to which one is the boot drive. You can choose the startup disk in the Preferences pane.
You can change the startup disk by formatting the new drive with GUID in Disk Utility.
I think the Bootcamp partition must be on the same drive as the startup disk, so that it shows up when you press Alt during boot.
If you use an external SSD drive, Bootcamp can not start the installation on an external drive (eg. external Thunderbolt enclosure with SSD), however you can install on the internal drive and clone the partition.
BootCamp installation on a secondary internal drives may require cloning a partition, see this post.