Is this: https://github.com/pulumi/pulumi/blob/master/sdk/nodejs/stackReference.ts#L21 would be the tool what does this?

Yes, StackReference does what you need. Here's an example where we created the identity stack separately from the infrastructure stack: https://github.com/pulumi/kubernetes-the-prod-way/blob/34c1bfded93822e02fb2ce8365f71b50b2d2d537/gcp/infrastructure/config.ts#L20