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.