J'ai deux musiques dans ma vidéo: elle commencent en même temps et non l'une après l'autre

In the editor you can add up to 2 audio tracks that will play simultaneously. It is meant to add a music track and a voice over for the video. You cannot have two music tracks playing back to back.

A great solution is to combine two videos, each with its own soundtrack, into a third video. Take a look at this blogpost, it explains the process in detail.
Another option is to loop your audio. You can find this option by clicking on the down pointing arrow (right side of the green bar).