Follow-up: Fixed.
I kind of feel stupid now, too. :-\
I submitted a Support Request last night. Support called today and had me fixed in 30 minutes.
I had my VM Display settings set for both Window and Full Screen to Stretch. What fixed things for me was changing just Full Screen to Resize.
The result is that in Full Screen mode, the VM sees both monitors and runs at the full display of both.
When I change back to Window mode, the VM continues to run in the same resolution (as if it has 2 monitors connected). Which is just what I want. As a window, everything is really small (unless I make the window big enough to almost fill both monitors. But, it doesn't cause all my open windows to resize and/or rearrange because the machine's resolution changed. Which, again, is just what I want. When I get a bunch of windows open and all positioned just so, I don't want them moving around every time I leave Full Screen mode - unless there was a way to make them automatically go back once I go back to Full Screen mode - which I don't think there is.
Anyway, happy to be fixed now. And I have no idea why I had to do this now, when I never did have to have my settings this way before and I was able to use Full Screen and Window mode like I want (i.e. with both set to Stretch).