One generator has two distinct frequency outputs. Here's why it is hard to fully answer your question with a hard number.
Generally, both outputs are used to generate the same frequency, with Out 2 used to double the voltage output. This results in four times the power for contact mode application.
One can generate distinct frequencies from both outputs, increasing the number of frequencies generated to two. However, Out 2 is generally mathematically related to Out 1 or static for all frequencies being generated on Out 1.
With some advanced programming methods, you can get two sets of specific frequencies on each output over the course of a program. See Bryan's work for tools that help in this method.
Finally, depending on the waveform used, you can generate multiple frequencies at once on each output, but again the subsequent frequencies are going to be mathematically related to the primary frequency.
Essential Oils (Harmonics) is an example where 8 frequencies are being generated all together based on the waveform used. Adding the 11th Harmonic is another example of doubling up two frequencies on the same output.
As to which frequencies not to mix, I know of none. There are tons of people out there running multiple generators with all manners of frequency combinations taking place.
Suggest that you start with the simple and work your way up to complex programs if you are trying to maximize a single generator. This way it becomes easier to identify what setting results in what output.
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