How do I set up private networks with static IPs on two onboard network controllers?

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http://askubuntu.com – I have found (and tried to solve for the past 6 hours) an intriguing situation.... I have a new install of ubuntu server 12.04 on fairly old hardware (core duo, two onboard network controllers). I have 2 networks that are connected but usually kept separate. one is 192.168.10.x and one is 192.168.5.x. If I set up both of them i can ping computers in the 10.x network, but not in the 5.x network. If I only set up the 5.x one I can NOT ping anything. If I set up only the 10.x one I can ping everything (even 5.x computers). curious side note, if i set the netmask to /16 i can't ping anything. i ca (HowTos)