I am planning to share the same AWS RDS (SQL) for 2 different sets of Sitecore databases. So I will have 2 Webs, 2 Cores and 2 Masters DB. Could anyone confirm if this is possible? Why or why not? The most important thing is this a good idea? The goal is cost saving while achieve blue/green deployment model for zero downtime.
You can install as many as you like, just have to be different database names.
As for AWS, you can't restore a single database in the RDS instance. You can only restore the whole instance. So keep that in mind.
Yes you can install as many Web, Master, Core Db instances on Sql Server just the names need to be different, and then according to whatever the names are and wherever the use is, you need to update the connection strings.