At what point can a host control another host in Westworld?

In the episode "Trace Decay".

Maeve convinced Lutz to modify her core code, enabling her to harm humans and to verbally control other hosts the way only human technicians are typically able to do.

In previous episodes, we see Maeve convincing techs to amend her abilities / core attributes to unprecedented levels...eventually grabbing the tablet and doing it herself.

Comicbook.com

In addition to tweaking some of her other stats, Maeve had the techs up her "bulk apperception" stat to its maximum level. One of the techs briefly explained in the episode that apperception was a measure of intelligence, but it actually has much deeper implications for the show's slowly building plot of the hosts becoming truly self-aware.

Apperception actually has nothing to do with intelligence, but rather a person's ability to make sense of an idea or event in the context of past events in that person's life. In layman's terms, it's the ability to understand something by comparing it to past experiences. People appercieve things in different ways based on their experiences, which makes it a fundamental part of how humans think and process ideas.