The SendGrid\Mail class supports adding multiple to addresses through the SendGrid\Personalization class.

You can see an example here: https://github.com/sendgrid/sendgrid-php/blob/master/examples/helpers/mail/example.php#L31-L35

Think of a Personalization as the envelope for your email. It holds the recipient's addresses and other similar data. Each Sendgrid\Mail object, must have at least one Personalization.

Through the constructor you are using, a Personalization object is already created for you, see here: https://github.com/sendgrid/sendgrid-php/blob/master/lib/helpers/mail/Mail.php#L951-L958

You can create a Mail object without this and later add your own Personalization.

In the end, this is how I have managed to do this and it's working good.

function sendEmail($subject, $to, $message, $cc)
{
    $from = new SendGrid\Email(null, "sample@email.com");
    $subject = $subject;

    $to = new SendGrid\Email(null, $to);
    $content = new SendGrid\Content("text/html", $message);
    $mail = new SendGrid\Mail($from, $subject, $to, $content);

    foreach ($cc as $value) {
        $to = new SendGrid\Email(null, $value);
        $mail->personalization[0]->addCC($to);
    }

    $apiKey = 'MY_KEY';
    $sg = new \SendGrid($apiKey);

    $response = $sg->client->mail()->send()->post($mail);
    echo $response->statusCode();
 }

function makeEmail($to_emails = array(),$from_email,$subject,$body) {
    $from = new SendGrid\Email(null, $from_email);

    $to = new SendGrid\Email(null, $to_emails[0]);
    $content = new SendGrid\Content("text/plain", $body);
    $mail = new SendGrid\Mail($from, $subject, $to, $content);
    $to = new SendGrid\Email(null, $to_emails[1]);
    $mail->personalization[0]->addTo($to);

    return $mail;
}

function sendMail($to = array(),$from,$subject,$body) {

    $apiKey = 'your api key';
    $sg = new \SendGrid($apiKey);
    $request_body = makeEmail($to ,$from,$subject,$body);
    $response = $sg->client->mail()->send()->post($request_body);
    echo $response->statusCode();
    echo $response->body();
    print_r($response->headers());
}

$to = array('test1@example.com','test2@example.com');
$from = 'from@example.com';
$subject = "Test Email Subject";
$body = "Send Multiple Person";

sendMail($to ,$from,$subject,$body);