How to declare a shared resource among different services in Serverless Framework

I have two services (each has its own serverless.yml file). Both services has functions that need access to a shared resource (e.g S3 bucket). What is the proper to define it in the serverless.yml files?

you can create s3 bucket resource in serverless stackA, set output key

Then in Serverless stackB, you can reference it

Details in document Reference CloudFormation Outputs