If you want to use two mp3 files playing at the same time in your video you can do so, even though it will be a bit of a workaround requiring you to generate a full version twice.
This is how you do it:
- Generate the video you have with your first uploaded mp3 file in "full".
- Download the video to your computer.
- Open a new blank storyboard and upload the downloaded video as an asset
- Add a "player slide" and upload your video as an asset.
- Under duration, choose "custom", and you are now able to set the duration in milliseconds.
- Select "audio controls" in the side menu and choose "mix music and main asset audio"
- Go to “Music” in the top menu and select a song.
- Generate this video in "full" version.