A basic SAM Template for an API that uses DefinitionBody and multiple OpenApi specs

apigwMultiApiSpecs.yaml

# Create a HttpApi with a YAML OpenAPI Spec
# Use Tranform Includes to dynamically import parts of the API via OpenAPI Specs

Type: AWS::Serverless::HttpApi or #AWS::Serverless::Api (REST APIs)
Properties:
  DefinitionBody:
    openapi: 3.0.1
      info:
        title: "myAPI-splitspecs"
        version: "2019-02-07T22:41:10Z"
      paths:
        'Fn::Transform':
          Name: 'AWS::Include'
          # Replace <bucket> with your bucket name
          Parameters:
            #Location: ./restapi-openapi.yaml  # s3://<bucket>/openapi-apigateway.yaml -- use this to pull from S3
            Location: ./httpapi-openapi.yaml  # s3://<bucket>/openapi-apigateway.yaml -- use this to pull from S3
        'Fn::Transform':
          Name: 'AWS::Include'
          # Replace <bucket> with your bucket name
          Parameters:
            #Location: ./restapi-openapi.yaml  # s3://<bucket>/openapi-apigateway.yaml -- use this to pull from S3
            Location: ./httpapi-openapi2.yaml  # s3://<bucket>/openapi-apigateway.yaml -- use this to pull from S3