How can a cloudformation resource refer to another resource in the same nested template?

Question

The CFN template below shows how resources in the nested template can refer to a resource from the parent template, but cannot directly refer to other resources in the same nested template. Is there a way to make these references work?

AWSTemplateFormatVersion: '2010-09-09'
Resources:
  OuterA:
    Type: AWS::S3::Bucket
  TestStack:
    Type: AWS::CloudFormation::Stack
    Properties:
      TemplateBody:
        AWSTemplateFormatVersion: '2010-09-09'
        Resources:
          InnerA:
            Type: AWS::S3::Bucket
          OuterB:
            Type: AWS::S3::Bucket
            # this works
            BucketName: !GetAtt OuterA.BucketName
          OuterC:
            Type: AWS::S3::Bucket
            # this works
            BucketName: !Ref OuterA
          InnerB:
            Type: AWS::S3::Bucket
            # Fn::GetAtt references undefined resource InnerA
            BucketName: !GetAtt InnerA.BucketName
          InnerC:
            Type: AWS::S3::Bucket
            # Unresolved resource dependencies [InnerA] in the Resources block of the template
            BucketName: !Ref InnerA

Answer 1

From AWS::CloudFormation::StackSet Documentation

Dynamic references in the TemplateBody may not work correctly in all cases. It's recommended to pass templates containing dynamic references through TemplateUrl instead.

In other words, to do what you want to do you should have a separate file for the nested stack, using TemplateUrl instead of TemplateBody, and pass in anything you need from the parent stack as parameters. This will allow references within the nested stack to work as expected.