forward alternate hostname to internal server

As you want to rewrite the host rather than the URI, your only option here is an iRule I'm afraid. If you wanted to redirect or rewrite the URI a HTTP Class would have done it.

You'd need a rule like so;

when HTTP_REQUEST {
 if { [string tolower [HTTP::header value Host]] equals "publicfqdn1" } {
  HTTP::header replace "Host" internalfqdn1
  return }
 elseif { [string tolower  [HTTP::header value Host]] equals "publicfqdn2" } {
  HTTP::header replace "Host" internalfqdn2
  return }
}