Got Coherence working, sort of
I just discovered that if I move the Parallels window to my MBP's built-in screen then enter Coherence mode, it works fine. If the window is on the second monitor, then entering Coherence mode makes the main monitor go partially black and the Parallels window disappears.
If it makes any difference, my main monitor is to the left of and slightly below the second one; see attached screenshot.
I just discovered that if I move the Parallels window to my MBP's built-in screen then enter Coherence mode, it works fine. If the window is on the second monitor, then entering Coherence mode makes the main monitor go partially black and the Parallels window disappears.
If it makes any difference, my main monitor is to the left of and slightly below the second one; see attached screenshot.