Many think they are. In fact, that's how they often look on paper after an audiological exam (if you have excellent hearing, the graph will look the same for both ears). But this doesn't mean the left and right ear process sound in the same way. Tomatis was the first person to observe that the right ear processes higher frequencies faster than the left. He also observed that there is a big difference between listening and hearing. Both observations have profound implications for the singer and voice teacher, especially as mixed-dominance is concerned.
Most people "lead" with their right ear, the very same ear that processes higher frequencies faster than the left. Practically speaking, /i/ and /e/ are right-ear oriented sounds, while /o/ and /u/, are left-ear oriented sounds. What does the highly skilled singer do? Learn to sing /o/ and /u/ with the ring of /i/ and /e/. Or course, this doesn't always happen, especially for the mixed-dominant singer, who can exhibit peculiar vocal behaviors, one of them being the tendency to lead with both ears. I've seen this repeatedly in the studio. The mouth of the mixed-dominant singer often points towards the right ear for /i/, and then towards the left for /u/, which creates a doppler effect in the vocal line. Getting the vowels all lined up? For the singer who goes back and both between the left and right ear, this means spending a very long time singing /e/ and /i/ correctly, then transferring the acquired acoustical awareness to /a/, then /o/ and /u/, a process which involves audition rather than overt manipulation. One problem however, is that the student's psychological framework—being linked to their manner of audition—is all over the place. Creating the environment for the necessary work takes great skill and patience.
I will say this for the mixed-dominant singer: once the transition is made, it is not forgotten. Singers who lead with the right ear as a matter of course often takes the process for granted. It's so easy for them, they hardly know what they know.