Hello Scott,
It wouldn't be possible to move the mouse outside of Windows' full desktop area. If set in Windows to "Extend these displays" the Windows desktop area (rectangle) would span your two monitors, and maybe with an extra rectangle if your two monitors are different sizes, so that Windows can present the entire desktop as a single rectangle encompassing both. Since you would only be able to drag the control panel's handle to wherever the mouse is, the handle must still be within Windows' full desktop rectangle area (even if not currently within the portions currently visible on the two monitors).
Before trying more drastic options (e.g. resetting things, or temporarily changing the Windows screen topology to force it to become fully visible again), the quickest and easiest option might actually be just to try to estimate how far the handle is from the edge of the visible monitor area, then try to move the mouse 'blind' that distance and drag it back. Since the handle is quite thin it might take a few attempts and a bit of practice, but I suspect only a couple of minutes or so.
Are you able to fix it that way?