To generate a Site Based Attendance Letter to a parent that doesn't live with the student: 

Step 1: Ensure the Contact record for the parent that does not live with the student has the Copy of All Mail mail tag associated. (CON.MT)

Step 2: When running the Site Based Attendance Letter, select the Print to Contacts option

• This will ensure that any contact for the student that has the Copy of All Mail mail tag will have a letter generated

Without selecting the Print to Contacts option on this report, the letter will only be generated for the parent and address listed on the Demographics page.