I have never had one serverfarm's servers need access to another serverfarm's servers only the same through the VIP but I believe the same principles apply. Basicaly you need to do client NAT as follows. You have to configure 2 different vservers, for farm 2 one for your regular client traffic and than another specifically for anything coming from the server vlan. Natting the client forces the reply back through the CSM instead of directly, which it would normally do because it is on the same IP segment.
vlan 10 client
ip address 10.10.20.10 255.255.255.0
!
vlan 20 server
ip address 10.10.10.10 255.255.255.0
natpool serverfarm1 10.10.10.111 10.10.10.111 netmask 255.255.255.0
serverfarm farm-1
real 10.10.10.1
real 10.10.10.2
vserver farm-1
virtual 10.10.20.100 tcp 0
serverfarm farm-1
serverfarm farm-2
10.10.10.3
10.10.10.4
serverfarm farm-2a
nat client serverfarm1
10.10.10.3
10.10.10.4
vserver farm-2
virtual 10.10.20.200 tcp 0
serverfarm farm-2
vserver farm-2a
virtual 10.10.20.200 tcp 0
vlan 20
serverfarm farm-2a
