Leica screw mount 2 and 3 series?

"Never" sums it up quite nicely I think, even the final IIIg variant had separate windows. The M-series were the first with one combined rangefinder/viewfinder window.
 
Canon made the transition to one window. They also added variable magnification for accurate focusing.

It was the Leica IIIb where the rear windows got so close that they are in one bezel, and might be mistaken for one window.
 
The Leica IV combined the RF/VF. It existed as a prototype, circa 1934. I suspect that patents held by Zeiss prevented it from going to market. The Fed 2 looks a lot like it.
 
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