@AviZ:
Your question is well beyond the scope of the DirectAdmin control panel. You can't send email to two servers; the SMTP protocol isn't designed for that. You can set up a backup MX server (though you'd need to do it yourself; it's not supported by DirectAdmin). In order for your users to be able to pick up email from a backup server when the main server isn't available you'd need either some kind of system for changing DNS on the fly when servers become unavailable or you'd have to have each user set up his/her own email client to read email from multiple servers.
All of this is well beyond the scope of DirectAdmin.
Jeff