You need to monitor resources that are distributed over different projects in Google Cloud Platform.

When you intially click on Monitoring(Stackdriver Monitoring) it creates a workspac(a stackdriver account) linked to the ACTIVE(CURRENT) Project from which it was clicked.

Now if you change the project and again click onto Monitoring it would create an another workspace(a stackdriver account) linked to the changed ACTIVE(CURRENT) Project, we don't want this as this would not consolidate our result into a single dashboard(workspace/stackdriver account).

If you have accidently created two diff workspaces merge them under Monitoring > Settings > Merge Workspaces > MERGE.

If we have only one workspace and two projects we can simply add other GCP Project under

Monitoring > Settings > GCP Projects > Add GCP Projects.

https://cloud.google.com/monitoring/settings/multiple-projects

Nothing about groupshttps://cloud.google.com/monitoring/settings?hl=en