When it comes to transmitter, amplifier or balanced modulator tests, you need a device generating two nonharmonically related  frequencies in the audio band : this equipment is known as a two-tone test generator.
The frequencies used in this generator are around 720 and 1890 Hz, located well inside the audio passband of any SSB circuit. The two Wien oscillators, designed to have a low harmonic spectrum, are presented in the  schematic diagram.

The outputs of the two oscillators are combined in an audio frequency mixer which gain is continuously adjustable. The resulting signal is applied through a switchable attenuator to a buffer driving two outputs (low Z and Hi Z).

It is important to properly tune each oscillator independently while checking its output level and harmonics content : you can use your PC sound card and one of the several freeware spectrum analyzer programs available on the Web. Then check the mixer output and adjust R11 or R18 if the two tones do not have the same levels ( this is the reason why R11 is paralleled with R11B on my circuit).

 ==> During these adjustments it is important to avoid clipping in  the audio circuit of the sound card.

Spectrum measurements conducted on my two-tone generator are summarized below :
 
 

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