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Yes, I caught the issue that this chip is designed for two-tone FSK. However, after thinking about your idea, it is not clear that this would work without knowing the undocumented internals of the chip. Very specifically, it is not clear that if both F1 and F2 hold the same frequency, that there would be no phase discontinuity when flipping the FSK input.
So, in the method you propose, at some point in the operation, set F1=F2 and change FSK. What should be the setting for ramped or non-ramped? If ramped, what rate? And then, the big question, how well is phase continuity guaranteed? This phase continuity is not clear to me from the chip specs.
This is a good and interesting topic.