You can add multiple speakers for videos longer than 2 minutes by giving a prompt as shown below and then clicking on Generate a video to continue.

You can also do this by selecting a workflow. In this instance: YouTube explainer.

After which you can click on the + sign to add multi-speakers to your video and add up to 5 speakers with 1 narrator.

For example: If you choose a Male or Female Clear American voice narrator, you have the option to add up to five additional speakers who have different accents.

These additional speakers will be able to speak the same language as the narrator.