Is the IP address for both servers the same? If so then you would need to put both web sites on the same web server and use host headers, if the IP addresses for the two servers are different and you want to use two separate web servers then you just assign the ip's to the astaro external interface and DNAT port 80 for each IP to the internal IP's of each web server, last thing to do is setup a packet filter rule to allow port 80 traffic to each of those servers.
