Hywel,
Give the 2nd NIC ip adres 192.168.100.100
The 1st NIC Ip adres is 192.168.1.1
The server that needs to run over the 2nd backupline must have and 2nd ip adress in the .100. range. Ie 192.168.100.101. You can put multi ip adresses on 1 nic.
When you change the host file on the backup server, it will always use the 2nd backupserver nic for that server, but it uses the 1st nic for every other server.