The exact definition of dominant frequency?
As dominant frequency it is usually meant the one that carries more energy w.r.t. all the other frequencies in the considered spectrum. 

Since in the first example  the two frequencies have the same absolute value I suppose they are both dominant frequencies. However the concept is not really important in these case, since you are just looking at the combination of two pure sinusoids of equal amplitude. 

In the second case, the lowest frequency is both the fundamental and the DOMINANT frequency since it has the highest peak of the considered spectrum.

Reference: https://arxiv.org/pdf/1306.0103.pdf