Please describe what you are trying to accomplish. It's not clear why you would want the same data(?) in two different tables.
I have different kinds of data that I would like to have in separate tables. Each record of the primary table will have a parallel record with different kinds of information. ie General information on the primary record and products purchased say in a secondary table. But I know that I will be need both types of records when I set up the primary record and so want the parallel record to be started when I establish the primary record. There is enough information in each type of record that it would be visually overwhelming.
But what problem do you solve by that? You can do it, but it's quite simple in FileMaker to add related records on an "as needed" basis instead of creating both records at the start.
but it can be done:
TableA::_pkTableAID = TableB:_fkTableAID (enable "Allow creation of records via this relationship" for TableB.)
#Starting on a Layout based on TableA:
#Create TableA record
New Record Request
#Create on related TableB record
Set Field [TableB::_fkTableAID ; TableA::__pkTableAID ]
This creates one and only one related record in Table B.
There are other scripts that can be used that create multiple TableB records all linked to the current TableA record.