Multiple databases in same AWS RDS PostgreSQL instance
I believe you can. Based on AWS RDS FAQs:


  Q: How many databases or schemas can I run within a DB instance?
  
  
  RDS for Amazon Aurora: No limit imposed by software
  RDS for MySQL: No limit imposed by software
  RDS for MariaDB: No limit imposed by software
  RDS for Oracle: 1 database per instance; no limit on number of schemas per database imposed by software
  RDS for SQL Server: 100 databases per instance
  RDS for PostgreSQL: No limit imposed by software
  


You can create additional database by connecting to you DB instance and do a CREATE DATABASE. Check this guide


  To create additional databases, connect to the DB instance and use the SQL command CREATE DATABASE. 


And the blog post on how to create new database in an existing PostgreSQL DB Instance:

psql --host=SOME-DBMS-HOST --dbname EXISTING_DB \
     --username=YOUR-USERNAME --password \
     --command="CREATE DATABASE new_database WITH OWNER some_owner"