Injecting more than one properties file into a Jenkins job
There is a simple way to get around the limitation you have.
You should load each file in the Build section, as a build step.
Use the Inject environment variables build step, and load each file you want. You can add multiple files by setting up multiple build steps of this type.
This works well for me on a similar need.
You can use Config File Provider Plugin to config some shell scripts.You can add multiple files and then execute them.