you will need to use the "sorry server" feature in CSS to acheive that
Sample Config
!********* SERVICE *****************
service serverA
ip address x.x.x.1
active
service serverB
ip address x.x.x.2
active
!********** OWNER ****************
owner SYED
content EXAMPLE
vip address 1.1.1.1
port 80
protocol tcp
add service serverA
primarySorryServer ServerB
active
********************************
HTH
Syed Iftekhar Ahmed