Identity Mapping will support multiple Identity Mapping Entries , please see below .
So what you can try is
1) create a new Data source for the on premise DB
2) Create a User Dir Object for the On premise DB
3) setup the Identity mapping as follows and let us see what you get
Create Identity Mapping that has multiple Entries as seen below with DIR B example being your RDS DB and On premise DB being DIR A in the example below
Link the Identity Mapping Auth Validate to your Realm .
You can see within my test Domain I do not have DIR A and DIR B listed
In my protected Realm , I have selected the Validation Mapping created above and saved the changes
After saving the changes , If I go back to my Domain and create a Policy , you can see that now Under the user Policy tab both DIR A and DIR B showed up from the mapping