Cmd-` with full-screen windows on one monitor
To summarize the set of keyboard shortcuts with the addition of @soulshined's input:


If I have two windows from different apps I can switch with ⌘ Command-Tab.
If I have two windows from the same app I can switch with ⌘ Command-`.
If I have two full-screen windows from the same app then ⌘ Command-` doesn’t work, but I can switch with ⌃ Ctrl-← or → (or ⌃ Ctrl-← or →; I chose ⌃ Ctrl-← or → to
avoid conflict with browser back/forward shortcuts).
If I have two full-screen windows from the same app on different
monitors then ⌃ Ctrl-← or → doesn’t work but ⌘ Command-` does.
If I have two full-screen windows from the same app on different
monitors and the target window is currently not in front then ⌘ Command-` and ⌃ Ctrl-← or → both fail to work. I have to ⌘ Command-Tab to another app that is on the target monitor, then ⌃ Ctrl-← or →  to the target window. Correction: ⌃ Ctrl-← or → scrolls through spaces on the original monitor, not the target monitor, so there's actually no way to do this that I know of.


Aside: if I enable the keyboard shortcuts for “Switch to Desktop 1” and “Switch to Desktop 2” and use two monitors, the shortcuts won’t work if the specified desktop is not assigned to the current monitor.

I've entered a usability bug with Apple, number 19363102.