Notice:
You have to change some configuration files becaus the because
the configuration of network interfaces must not be equal.
!! Take care, to use the right device names !!
Maybe they are called i.e. 'enp0sXX', but you can rename it.
See also : README.rename.netdevices
For the backup gateway host:
eth1 --> LAN
eth2 --> WAN or ppp0 (DSL device)
eth0 --> WLAN or second LAN or what ever
or
br0 --> WLAN or second LAN or what ever
So you have to change the following files
dsl-provider.ReachOut: ppp0 comes over eth2
interfaces.ReachOut: see above
default_isc-dhcp-server.ReachOut
ipt-firewall.ReachOut: LAN device (mostly ) = eth1
second LAN WLAN or what ever (if present) = eth0