TMG --> Big IP migration

I'm not completely familiar with TMG but in the brief reading I did here, it sounds like what you're looking for is the ability to have a single virtual server (with 1 IP address) process traffic for multiple HTTP applications. For example, and both resolve to a single IP address, but you want those requests for to be directed to one back end application server while those requests for to be directed to a different back end application server. On a BIG-IP, you could have a single virtual server that listens at the IP address DNS resolves to for both names, then use an iRule to select the appropriate back end server based on the host name requested. Your iRule might look something like this:

when HTTP_REQUEST {
    switch [HTTP::host] {
        "www.app1.com" { node 172.16.20.1 }
        "www.app2.com" { node 172.16.20.2 }
        default { node 172.16.20.3 }
    }
}

Depending on how many applications are supported by the single virtual server, you could change from using a switch statement to using what's called a "data group" where you keep a mapping of host names and associated IP addresses.