In a very early incarnation of this circuit, the motors would take two steps in response to a single cycle control pulse. That happened because, even though the sine signal was only switched on for a half cycle, the cosine made a polarity change during that half cycle. Eventually, I realized that I needed to trigger the switches whenever either the sine OR the cosine made a zero-crossing. Another pair of comparators and some diode-transistor logic was added to the circuit, and single stepping became the norm.

A few weeks ago, a thread on .Stack reminded me of a fun bit of analog hardware called the ring modulator. It multiplies ine input signal by the other input signal, for a loose definition of multiply. Mostly, it is just interpreting the signs (not sines) of the inputs (sines). Whatever it does, the resulting output signal resembles a sine wave at twice the input frequency. With twice as many zero crossings, I need half the comparators. Out with the 339 quad, in with the 393 dual!

I'll modify my build the next rainy day, then post an update. Stay tuned!