This is an old thread but ran across it and wanted to post for anyone finding it still. Simplest was to combine the copying mentioned above in the package.json. In the root of my folder I have two serverless files, serverless-public.yml and serverless-admin.yml and then the two scripts just pull one and copy it:
"start-dev-public": "rm -f serverless.yml && cp serverless-public.yml serverless.yml && sls offline start --stage=dev --port=4000",
"start-dev-admin": "rm -f serverless.yml && cp serverless-admin.yml serverless.yml && sls offline start --stage=dev --port=4999",