Simultaneous synthetic vowels can be separated by their fundamental frequencies alone.

Evidence

In an experiment that bridges speech perception and auditory perception, a doublevowel task has also illustrated that listeners benefit from f0 differences in speech-like stimuli. In this type of experiment, a listener is presented with two simultaneous harmonic sounds, which each has formants modeled after natural vowel sounds. The two synthetic vowels can have the same or different fundamental frequencies, and a listener identifies the two vowels in the double-vowel pair. — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 1320