Drag the audio into the main editing pane of Soundbooth. This will copy the file over and ready it for editing.
Right-click and select "Insert Channels into New Multitrack File." This will create two channels, a left audio channel with your original audio and the right audio that is empty. In the newer version of Sounbooth CS4, it will copy the original channel into the right channel as well. If you have CS4, this is your last step.
Copy the left channel by clicking on it once to select it. Then right-click and select "copy."
Place the cursor at the beginning of the audio file in the right channel. Press "CTRL-Z" or right-click and select "paste" to copy the channel over.