With a standard pfSense you won't be able to do it.
If both websites are on the same physical server, running under the control of a single web server (eg. Apache), then virtual host configuration will be able to handle it.
If you have physically different servers (eg. 192.168.1.2 and 192.168.1.3), then you will have to set up some kind of proxying facility. You can use a third box (ugh), or install something like ffproxy, varnish, or mod_proxy on Apache on one of the servers.
If you go for the second option (recommended) then you'll forward all incoming HTTP requests to this box and the proxy will have to analyse the "Host" header on the incoming HTTP requests and route it to the relevant web server.