CodeBuild - multiple input artifacts in AWS Pipeline
We've had exactly the same problem - our build stage needs to use information from two separate locations.

To solve this in a generic manner, I've created two general purpose Lambdas - one that will merge artifacts, and one that will extract specific files from an artifact. In both cases a new artifact is created that can be passed on to a subsequent CodePipeline stage.

See https://github.com/tooltwist/codepipeline-artifact-munge

If it does not meet your exact requirement, it should be easy enough to tweak to do what you need.