Mos flip-flop hubs have one stepped-thread for fixed (ie lockring) and one freewheel type thread.
You need 2x stepped-thread flip-flop hubs :
philwood
also
Goldtec
Formula

Chain tension is by horizonal dropout or track ends. Sliding vertical dropouts require re-setting each time you change gear so are better suited to hub gears.
You cannot use a spring tensioner for fixed.