One thing you have to remember is that the stereo tremolo has good deal of mono information on it, and that information is at double the stereo information. I'll try to explain this. Imagine you recorded a suitcase piano and mixed it down to mono. If there was no mono information in the signal, the tremolo would disappear. This would happen if the total sound power put out by the piano remained the same and the tremolo simply shifted it back and forth between the 2 speakers. The tremolo would not be useful if this was the case. Think, for a moment, of a stage piano in some performance venue. The suitcase speakers are quite close to each other, and if you weren't positioned close to the stage, you wouldn't be able to localize the speakers from each other. Additionally, as you move from the stage, you leave the near-field region and enter the far-field region where much of the sound you hear is that which has bounced off room boundaries. In these conditions, the piano effectively becomes a mono source, and once again, unless the tremolo contained mono information, the tremolo effect would disappear.
That the tremolo does not disappear means it does contain mono information. This information exists because as the sound is shifted from speaker to speaker, the total sound power varies. Specifically, this means that either there is more sound power put out when the 2 speakers are putting out an equal amount of power, or there is more power when one speaker is putting out the most power and the other speaker is putting out the least power. Either case is possible; it depends on the modulating waveforms that are fed to the 2 power amps.
Let's take the case in which most sound power is put out when either speaker is at the peak of its volume. Now, imagine you are sitting cross-legged on the floor right in front of the suitcase speakers. Suppose both speakers are at the point in the tremolo cycle in which they are putting out the same volume. Now say the left speaker starts to swell in volume, and then its volume diminishes and the right speaker swells in volume, and then its sound goes down until both speakers are putting out the same volume. You would perceive center, shift to the left, go back to center, shift to the right, and go back to center. You would consider and hear that to be one cycle of the tremolo.
Now imagine yourself far enough from the piano that the stereo information is no longer perceived. Under the same situation, you would hear the volume increase as the left speaker got most of the power, hear the volume go down as the speakers put out equal power, hear the volume go back up as right speaker reached its peak, and then hear the volume go down as the speakers. That would sound like volume up, volume down, volume up, volume down. You would consider and hear that as 2 cycles.
