Firebase cloud function with access to two different projects

I‘m currently developing two different web apps for university project, using Firebase for hosting, as database and Firebase cloud functions for some back-end work. I‘m using Firebase for the first time though.

Now, as the cloud function I have does the job for both projects, I do not want to deploy the function in both projects, but instead give the cloud function access to both projects.

Is there any way I can accomplish this?
What I tried so far was deploying cloud function to project A and there initializing two apps in my cloud function node.js environment, one with project A‘s firebaseConfig and one with project B‘s config. But I keep getting an error (permission denied: missing or insufficient permissions) when I tried to access the Firestore database of Project B. There are no specific rules set for database access as still in development, so that can‘t be the problem.
Can someone help me out?

Best regards!

Given:

• Project A
  • Service Account A
  • Cloud Function A running under the context of Service Account A
• Project B
  • Firestore B

If you want to have Service Account A access Firestore B then you must grant Service Account A access to Project B, then after it has been granted access you must assign Service Account A a role that gives it permission to access Firestore B, for example:

• Firebase Admin (if you want to grant it broad permissions)
• Cloud Datastore User (if you only need it to read/write Firestore data)

Without proper IAM permissions in place you cannot have cross-project access.