So to answer OPs question: "How is this modeled in the buildspec "... its not... this can be modeled however using a JSON Configuration file for the Pipeline.

You cannot configure a build step with multiple actions with one or more buildspec files in a single codebuild project, nor can you use multiple codebuild projects in a build step when configuring an initial codepipeline from the GUI.

You will need:

• Multiple buildspec files.
• A codebuild project for each buildspec file.
• A customised json configuration file for the build step. ( This is the answer to OPs question )

You will deploy your codepipeline with the JSON configuration file to accomplish what you need.

See here for more information on this configuration file