if you have 1 router and 2 backbone, it means the router is attached to both.
Therefore, I would suggest to simply put the CE in one of the backbone and use wccp to transparently redirect traffic from both backbone to the CE.
For example, assume your 2 backbone are respectively on interface fast 0/0 and fast 0/1.
You will put the CE on interface fast 0/0.
On the router you will configure
ip wccp version 2
ip wccp webcache
interface fast 0/0
ip route-cache same-interface
! important because this is where the cache is located
ip wccp webcache redirect in
!
interface fast 0/1
ip wccp webcache redirect in
!
On the cache, use just 1 interface with 1 ip address from the backbone 1 range. Then configure
wccp router-list 1
wccp version 2
wccp webcache router-list 1
Hope this help.
Gilles.