I created a virtual eth1 dummy interface to test 
$ sudo modprobe dummy
$
$ lsmod
Module Size Used by
dummy 16384 0
$ sudo ip link add eth1 type dummy
$
$ sudo ip address add 192.168.2.2/24 dev eth1
$
$ sudo ip link set eth1 up
$
$ ip -br -4 address show scope global
eth0 UP 10.0.0.2/24
eth1 UNKNOWN 192.168.2.2/24
$ sudo pihole-FTL dhcp-discover
Scanning all your interfaces for DHCP servers
Timeout: 10 seconds
* Received 327 bytes from eth1:192.168.2.2
Offered IP address: 192.168.2.183
Server IP address: 192.168.2.2
Relay-agent IP address: N/A
BOOTP server: (empty)
BOOTP file: (empty)
DHCP options:
Message type: DHCPOFFER (2)
server-identifier: 192.168.2.2
lease-time: 86400 ( 1d )
renewal-time: 43200 ( 12h )
rebinding-time: 75600 ( 21h )
netmask: 255.255.255.0
broadcast: 192.168.2.255
domain-name: "network2.dehakkelaar.nl"
router: 192.168.2.1
dns-server: 192.168.2.2
--- end of options ---
* Received 318 bytes from eth0:10.0.0.2
Offered IP address: 10.0.0.33
Server IP address: 10.0.0.2
Relay-agent IP address: N/A
BOOTP server: (empty)
BOOTP file: (empty)
DHCP options:
Message type: DHCPOFFER (2)
server-identifier: 10.0.0.2
lease-time: 86400 ( 1d )
renewal-time: 43200 ( 12h )
rebinding-time: 75600 ( 21h )
netmask: 255.255.255.0
broadcast: 10.0.0.255
domain-name: "home.dehakkelaar.nl"
dns-server: 10.0.0.2
router: 10.0.0.1
--- end of options ---
DHCP packets received on interface eth1: 1
DHCP packets received on interface lo: 0
DHCP packets received on interface eth0: 1