The curtain idea is a good one, cheap and easy. But seemingly a little inconvenient.
You could hinge a piece of black board on both windows that could flip up depending on the window you wanted to use at the time. But, you'd be limited to one window at a time, which I don't think you want.
Or, if you covered the windows with perforated window film such as used by printers for advertising on windows and rear truck windows, you could see out of both sides at one time without being seen from outside - that way you wouldn't have a curtain getting in your way.