New Ubuntu Install: How to configure Dual Monitors

I just installed Ubuntu 15.10. I have a machine with two LCD monitors. Ubuntu displays the same thing in both monitors. I tried SystemSetting->Displays, but this complained about an inteal Ubuntu error, and then disappeared. I never saw the error message again.

How can I force Ubuntu to use my two monitors correctly (not mirror of each other)? In RedHat, the command was something like system-config-display. I searched the entire filesystem and could not find an obvious command to do this. What is the command to configure the monitors in Ubuntu 15.10?