Many think they are. 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 processes sound similarly.

Tomatis was the first to observe that the right ear processes higher frequencies faster than the left. He also observed 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 same ear that processes higher frequencies faster than the left. Practically speaking, /i/ and /e/ are right-ear oriented, while /o/ and /u/ are left-ear oriented. What does the highly skilled singer do? Learn to sing /o/ and /u/ with the ring of /i/ and /e/. This doesn't always happen, especially for the mixed-dominant singer, who can exhibit peculiar vocal behaviors, one of which is the tendency to bounce back and forth between the ears. I've seen this repeatedly in the studio—the mouth of the mixed-dominant singer pointing towards the right ear for /i/ and then towards the left for /u/—creating a Doppler effect within the vocal line.

Getting the vowels all lined up? For the singer who goes back and forth 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—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: it is not forgotten once the transition is made. Singers who lead with the right ear, as a matter of course, often take the process for granted. It's so easy for them, they hardly know what they do.