Are the dbs on render isolated?
Meaning is it its own pod on the cluster or is it a shared RDS with multiple DBs?
If I have 2 db instances can the communicate between them without going out?
DBs run as their own services, yes, their own pods, independent of application services (Web Service, Private Service, Background Worker).
All Services in the same team and hosted in the same region have a Private Network ( https://render.com/docs/private-network ) that allows them to intercommunicate without going over the public internet. This is why Database services have the “Internal Database URL” and “External Database URL” rows on the Render Dashboard.