Using iproute2 you can do something like this:
1 echo "1 admin" >> /etc/iproute2/rt_tables
2 echo "2 users" >> /etc/iproute2/rt_tables
3
4 ip rule add from 192.168.122.40/32 dev eth0 table admin
5 ip rule add from 192.168.123.41/32 dev eth1 table users
6
7 ip route add default via 192.168.122.1 dev eth0 table admin
8 ip route add default via 192.168.123.1 dev eth1 table users
Now you will have two routing table with two gateways.