Un moyen simple d'envoyer 2 emails en 1 en PHP, grâce au Content Type multipart/alternative. Si le client mail du destinataire gère le HTML, c'est cette version qui sera affichée, sinon ce sera la version texte.
Vous me direz qu'aujourd'hui, tous les clients mails gèrent le HTML. C'est vrai mais certains utilisateurs le désactivent volontairement.

Je me fait une copie du code donné en exemple dans l'article :

//specify the email address you are sending to, and the email subject
$email = 'email@example.com';
$subject = 'Email Subject';

//create a boundary for the email. This 
$boundary = uniqid('np');

//headers - specify your from email address and name here
//and specify the boundary for the email
$headers = "MIME-Version: 1.0\r\n";
$headers .= "From: Your Name \r\n";
$headers .= "To: ".$email."\r\n";
$headers .= "Content-Type: multipart/alternative;boundary=" . $boundary . "\r\n";

//here is the content body
$message = "This is a MIME encoded message.";
$message .= "\r\n\r\n--" . $boundary . "\r\n";
$message .= "Content-type: text/plain;charset=utf-8\r\n\r\n";

//Plain text body
$message .= "Hello,\nThis is a text email, the text/plain version.
\n\nRegards,\nYour Name";
$message .= "\r\n\r\n--" . $boundary . "\r\n";
$message .= "Content-type: text/html;charset=utf-8\r\n\r\n";

//Html body
$message .= "
 Hello,
This is a text email, the html version.

Regards,
Your Name";
$message .= "\r\n\r\n--" . $boundary . "--";

//invoke the PHP mail function
mail('', $subject, $message, $headers);