Why: 14 CFR § 107.35 states: "A person may not manipulate flight controls or act as a remote pilot in command or visual observer in the operation of more than one unmanned aircraft at the same time."
Assumes a two-aircraft exception tied to visual line of sight, but § 107.35 does not provide one — the prohibition on controlling or commanding more than one aircraft at a time is absolute.
Assumes adding visual observers can satisfy the multiple-aircraft prohibition, but § 107.35 bars a single person from controlling or commanding more than one aircraft regardless of how many observers are present.