You can't. I'm assuming you only have one public Internet IP address. You cannot forward the same port to different machines. The only way to do it is to forward some other port, i.e. 81, to your second website.
Another way to do it is to set up one apache server that serves two name based virtual hosts. Both domainnames will point to the same IP address. This of course means both websites need to be served from the same host or jail.