The easiest way is to create a stereo track with one sound in the left channel and the other sound in the right channel.
To join two mono tracks to create one stereo track, see: http://manual.audacityteam.org/man/splitting_and_joining_stereo_tracks.html
You can then multiply the sound in the left channel with the sound in the right channel, and return the result to both channels:
(mult (aref *track* 0)(aref *track* 1))