Sorry, I misunderstood.
What you have should provide the user with subform rows each containing two text fields. The user can add additional rows by using the + button.
I'm not sure that you need the layout="..." bit, I think it should work without specifying a layout unless you want to change the default
By specifying that the subform has mulltiple="true" then it will render with the buttons for the user to add and delete rows, and reorder them. You can set a min and max number of rows, and also whether you want add, remove, move buttons for the rows using the field parameters.
I'm not sure quite what you are asking when you say "Question: how can I fill a string with standard values multiple times?". Which string, where? Are you wanting to pre-populate the text fields with default values? In that case you might need to write a field definition in the models/fields folder and reference that in formsource="..." parameter for the subform.
If you want 5 lines showing to start with then set min="5" in the parameters, If you don't want the user to delete rows then set buttons="add,move" or whatever buttons you do want - buttons="" will remove the buttons.
Sorry not to be more helpful
RogerCO