You can also add layers (overdub) to the audio track. When you select the audio record button, a window opens and to overdub select the overdub underlying audio button.
A further note. You can use the overdub feature multiple times. Therefore, you can add new instruments or do multiple harmonies if you wish just by continuing to record overdubs. The overdubs are merged rather than mixed so the quality is rather dubious.
However, if you are that ambitious with your productions, you will be better served to do those type things in RealBand. It will be easier and the results will be of a much higher quality.