I've tested this using the following powershell script and using (,) between the addresses. It worked for me!
$EmailFrom = "<[email protected]>";
$EmailPassword = "<password>";
$EmailTo = "<[email protected]>,<[email protected]>";
$SMTPServer = "<smtp.server.com>";
$SMTPPort = <port>;
$SMTPClient = New-Object Net.Mail.SmtpClient($SmtpServer,$SMTPPort);
$SMTPClient.EnableSsl = $true;
$SMTPClient.Credentials = New-Object System.Net.NetworkCredential($EmailFrom, $EmailPassword);
$Subject = "Notification from XYZ";
$Body = "this is a notification from XYZ Notifications..";
$SMTPClient.Send($EmailFrom, $EmailTo, $Subject, $Body);