If I have understood, you want to save the first_name, middle_name and last_name in one table and the full_name in another table.
I would do this in two steps :
- 1rst step : save the child table with the different names
- 2nd step : save the parent table
So 2 create() methods one after the other.
You have also the possibility to create a MySQL view (assuming you are using MySQL), so that the view automically updates when the other table is updated.