For transparent proxy, you can use WCCP or L4 switch to redirect web traffic to S-series and achieve load sharing and high availability. For WCCP setup information, you can find more information from:-
http://tinyurl.com/58rqk2
If you are using explicit proxy, you can make use of PAC file to assign two or more proxies for load sharing and high availability. More examples of PAC file can be found in:-
http://tinyurl.com/ycoodrp
You can also use load balancer as well.