How to set up two default routes in linux

Using iproute2 you can do something like this:

echo "1 admin" >> /etc/iproute2/rt_tables
echo "2 users" >> /etc/iproute2/rt_tables

ip rule add from 192.168.122.40/32 dev eth0 table admin
ip rule add from 192.168.123.41/32 dev eth1 table users

ip route add default via 192.168.122.1 dev eth0 table admin
ip route add default via 192.168.123.1 dev eth1 table users

Now you will have two routing table with two gateways.

This phenomena is called unicast rpf (reverse path forwarding) as a standard but reverse path filter in Linux. You may disable this without any important negative consequences (unless you're an isp.. and a router).

echo 0 > /proc/sys/net/ipv4/conf/default/rp_filter

Then your example should work as you want it to.