You only have 1 Public IP, correct?
If so, you cannot use the firewall with 1 Public IP (i.e.
only 1 port 80/tcp available) to somehow "split" it into two private IPs (i.e. somehow make 2 port 80's). You'll need a HTTP proxy that can use "see" the hostname and forward it to the correct server.
(You're literally describing why IPv4 NAT and port forwarding exists.)