PHP Form Mailer

I have created an e-mail form based off of the tutorial located at:

Do you know if it is possible for me to set up a form to include multiple e-mail addresses? Right now, the form I have set up sends mail to only one e-mail address. I tried putting an extra $to line below the first one that lists the e-mail address, but when I tested out the form, only one of the two e-mail addresses actually received the e-mail I sent through the form. Any help would be great.