Are you talking about one window where both sash slide or a twin unit with an integral frame? Crystal does make a sliding window where both sash operate. In aluminum and vinyl.
If you're talkin about combining two double hung Windows yes you can with a piece of plastic in the middle called a Mullen. But make sure there is some type of sealant or caulking when you do the window assembly especially in the Mullen I have seen situations on new buildings when they combined to double hung windows with that plastic molding strip and the water just seat right down from the top and into the bottom of the window frame and rotted away. Window sill and moldings especially if the builders used MDF board for the trim
Are you talkin about just one opening with two windows that slide because that would be considered in one window
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