Yes, it is possible. You can define one VIP to load balance traffic on your web servers and another VIP (on the same subnet) to allow load balancing between web servers and back-end servers. If the web servers use a different TCP port to communicate with the back-end servers as for the web access, you can even use the same VIP address and two content rules.
Something is however very important to make this working : you have to NAT the source address for the backend servers connections to make sure the return traffic pass through the CSS and not directly to the web server (they are on the same subnet).
Yves Haemmerli (IBM)