The dominant species lives in large groups. The lesser species is widely and sparsely distributed.
Your dominant species lives in settlements of dozens to thousands of individuals. They have agriculture. If you see one there are others nearby, and they work well in groups.
The ape-like species is spread widely through the land, including wild places and inhospitable places. They live off the land as hunters and gatherers. They live in family groups of a dozen individuals at most. They are not antisocial, but the more people there are in a group the harder it is to keep them all fed.
Where ape and human populations overlap the apes are there for the food, and are employed as farm laborers. The ape-like species is controlled by the more human species because the ape-like species is never present in any numbers and humans always are. If an individual ape acts up it is quickly overpowered by many humans. This is also the case if an individual human acts up. Offenders are punished, or banished, or executed as the crime warrants.