Strange, I think that should work.
Try with < and >.
$ToEmail = '<treelinemedia@hotmail.com>, <ian@ripzoneinternational.com>';
if that doesn't work, just do this:
$ToEmail1 = 'treelinemedia@hotmail.com';
$ToEmail2 = 'ian@ripzoneinternational.com';
mail($ToEmail1, $subject, $body, $headers);
mail($ToEmail2, $subject, $body, $headers);
It's not quite as efficient, but it gets the job done.