Under 14 CFR § 107.33(a), if a visual observer is used during an operation, the remote pilot in command, the person manipulating the flight controls, and the visual observer must —

Why: 14 CFR § 107.33(a) requires that "the remote pilot in command, the person manipulating the flight controls of the small unmanned aircraft system, and the visual observer must maintain effective communication with each other at all times."

• Invents a specific radio-frequency-separation scheme; § 107.33(a) requires effective communication among the crew, but does not prescribe any particular frequency arrangement.
• Assumes the visual observer role itself requires a remote pilot certificate, but § 107.33 describes the observer's duties as seeing the aircraft and coordinating with the crew, not holding a certificate.