Two subnets on one interface

Solution 1:

Try changing /etc/network/interfaces to

auto eth0
iface eth0 inet static
    address 192.168.1.10
    netmask 255.255.255.0
    network 192.168.1.0
    broadcast 192.168.1.255
    gateway 192.168.1.1
    dns-nameservers 192.168.1.1

auto eth0:1
iface eth0 inet static
    address 192.168.2.10
    netmask 255.255.255.0
    broadcast 192.168.2.255

Restart networking or reboot:

sudo service networking restart 

@2707974 's answer will produce:

SIOCSIFADDR: File exists
SIOCSIFFLAGS: Cannot assign requested address 
SIOCSIFFLAGS: Cannot assign requested address 

Sadly I don't have enough points to comment under his answer.

Updated my answer: @2707974 pointed out that my original answer was bogus -> see question.