That could be done, but if you double click the relationship line and select "allow creation of records via this relationship" for the new table, there is no need to do so. You can add the fields from your new table to a layout based on the original table and then the first time that a user enters data into one of these newly added fields, FileMaker will create the new related record for you to hold the data entered into the field.
Extending this question, the system only allows a key to be related to one other table.
If I wanted to add two or more subservient tables, I presume I need to create a duplicate index field in the main table.
What is the simplest way to do that?
How do I ensure the original key and the duplicate index field are kept in synch?
Do I have to take any action to populate the duplicate index field?