Why are there two phases for one dominant frequency?
If the dominant frequency is not exactly integer periodic in the FFT width, then most of that frequency peak's energy will be split between the two nearest FFT result bins (two different frequencies).  The phase of your frequency peak might be slightly ahead or behind the phase of the slightly lower FFT frequency bin. If so, it will have the opposite phase relationship (slightly behind or ahead) of the the FFT frequency bin slightly on the higher or opposite side in frequency difference.

So it's not two different phases.  It's two opposing phase relationships when your dominant frequency spectra is correlated against the two nearest frequency FFT basis vectors, that are both slightly different (and one integer period of the FFT length apart), but on opposite sides in frequency, and thus opposite in phase difference relationship to your dominant frequency peak.

Unless, of course, your dominant peak represents a signal that is exactly integer periodic in the FFT aperture, in which case only one FFT result bin (per harmonic) will be involved, and you won't see two phases (for any non-zero FFT result bins that are above the noise floor)