In the Keyframe Editor, you can view the audio tracks in your project, along with the Level and Pan curves for each track.
To create a crossfade, add keyframes to the level curve of each track at the same (or nearly the same) points where you want the crossfade to start and end.
Listen to the crossfade as you work. You may find that the best-sounding results are achieved by having the level curves for the two tracks be slightly asymmetrical. This is because the perceived loudness of different sounds with the same numerical value can be different.