I would think in the function send at line 552 (before line 554 when it closes the connection).
If not using SMTP then line 572. (for if SMTP failed... it tries to use PHP Mailer
Also function sendmail (line 590) starts if you do not use SMTP at all and have osTicket set to PHPMail. You would have to rewrite the function to send two mails.
But these are my best guesses since I haven't tried to do this before.