I was reading three different scifi magazines at the time, and buying dozens of those Ace Doubles. I wonder how many of yall are old enough to remember those Ace Doubles?.
Those are a little before my time, but I remember seeing them around, possibly in libraries and used bookstores. And Tor did a line of Tor Doubles back in the early nineties.
A brief digression: Tor recently revived the Doubles format to publish paired sf novellas by Walter Mosely. By sheer coincidence, I just happened to write the cover copy for next Double a few days ago . . .