Two things I can think of to check.
1) Make sure that on all the servers, it is the primary NIC which has been configured with a gateway address, and not the secondary NIC.
2) In the TCP/IP settings of the secondary NIC's, make sure you've deselected the "Register this connection's addresses in DNS', as if it is selected you'll end up with entries in your local DNS for the server using the secondary NIC IP Address, which would then cause the servers to communicate using the wrong network. Especially if you don't have DNS entries for the primary network.