Why A two-tone test feeds two non-harmonically related audio tones (for example 700 Hz and 1900 Hz) into an SSB transmitter and the RF output is examined on a spectrum analyzer or oscilloscope. If the amplifier chain is perfectly linear, only the two RF tones appear; any nonlinearity mixes them and creates intermodulation distortion products at other frequencies, which show up as extra spikes and as splatter on the air. The level of those IMD products relative to the two tones is the measure of linearity.
Watch out Carrier and unwanted sideband suppression is checked with a single tone, where you can see the carrier and opposite-sideband residuals as separate spectrum lines; two tones would just clutter that measurement.
Two tones in, only two tones out = linear. Extra spikes = IMD.