iMovie - Music Transition

You can overlap the two tracks instead of putting them back to back. You can move second track around to minimize the ramp up/ramp down between the songs. The second track needs to be inserted on a different timeline from the first track.

Make use of the 'Show audio wave forms' option to see where in the audio track the music actually stops so that you can start the next one closer to that point by overlapping them.

You can also use 'Show volume level' to achieve a nice fadeout of the sound of one track as you fade in the next one.

+The second track needs to be inserted on a different timeline from the first track.+

You can do it that way, but you can overlap them on the same track if you want or need to.