I see. If that's the case then I would suggest promoting the TTU ID field as a column for the Form Name (Go to File > Advanced Form Options > Property Promotion, then select the field). Then, instead of having the TTU ID field populating the Save As field, proceed to Form Load (located under the Data tab) and create an Action Rule which generates a File Name for the field Save As, and add a condition the this rule should only run if Save As does not have any value.
Save As = File Name (use this field at the SharePoint Library Data Connection)
TTU ID = Form Name (promote this field when publishing the form)
Your SharePont Library should now have a column for the File Name and another column for the Form Name. Note that the File Name should not change it's value even if the form has been re-opened, in order for the form to be overwritten when re-submitting. I hope this clears up everything for you, but if not, kindly attach your form so we could have a look.