Why A two-tone test feeds two audio tones into an SSB transmitter and looks at the RF envelope or spectrum for intermodulation products, which reveals amplifier linearity. The tones must not be harmonically related (the classic pair is 700 Hz and 1900 Hz) so that any distortion products land on frequencies clearly separate from the tones themselves. If the tones were harmonically related, the IMD products would fall right on top of the harmonics and you could not tell distortion from the test signal.
Watch out Two tones of the same frequency shifted 90 degrees describes quadrature or I/Q signals used in phasing modulators, not a linearity test; square waves are already full of harmonics and would swamp the measurement.
Two-tone test: two tones, not harmonically related, classically 700 Hz and 1900 Hz.