To adjust the volumes of the two tracks, select the envelope tool from the left top corner toolbox, and use it to click on the tracks where you want volume changes to occur. This creates a keyframe, represented by the small white dots that appear on the two blue lines. These lines represent the volume of the track: the farther they are from the center of the waveform, the larger and louder the wave grows, and the closer, the quieter.
To make the volume change, position the envelope tool cursor over the keyframe you created, and click and drag. The track gives you visual feedback on how loud or soft the wave becomes.