In case you have two linked records fields pointing to the same table, by default, they will share the same child form settings. If you want to have a separate child form setup for each of them, you need to do the following:
Click one of the Portal Table or Linked Records Field.
Go to Create & Expand Records section
You can use a form for creating and another form for expanding records.
Also besides the Edit Form button, there is another button.
A modal will appear where you can have the option to create a new child form.
After clicking the Add button, the portal table or linked records field already has a separate child form you can configure.