Virtual Server to load-balance two ISA Proxy servers

If your applications require authentication, I would recommend against it due to ISA requirements for bidirectional affinity. Also, ISA is limited to unintelligent monitoring and round robin load balancing of internal web farms. It would make more sense to have the F5 load balance the applications, or completely replace the ISA servers with LTM + APM.

You can LB ISA's quite easily. If they're acting as proxies, you just LB port 80 for proxy services. If you use an http profile then on v11 you'll have to disable http processing for the CONNECT method as well. (I use an http profile to enable the use of response logging.

I usually add a port 0 VS on the same VS IP address too, to catch the ISA client traffic (Kind of socks, but with the usual MS we'll make it different approach). Then use src-addr persistence with match across services.