Actually, you can play twin track stereo on a quarter track stereo deck, they are stacked the same way, just lower signal and a bit more hiss. Where you run into problems is early stereo tapes were staggered, not stacked. Foe example, the crosstalk on early attempts at stacked stereo heads was so poor that the only way to have a quality twin track tape was to stagger two mono playback heads, one upside down, like on Webcor mono decks that could reverse without flipping the reels. Akai made a deck that could play stacked, staggered, twin track, quarter track, mono and stereo. Too bad it was a mechanical deck like the Sony's and the head was softer too. Not too many survive and fewer techs to do mechanical overhauls.Getting idlers resurfaced or rebuilt is not cheap either.