In the days of the XYScope display platform, researchers often used a second, smaller XY oscilloscope to monitor the stimulus being displayed to the subject on the primary scope. Once the old analog scopes broke down and RMVideo became the primary stimulus display for Maestro, the Lisberger lab gave up on using a second monitor in this manner because the NVidia graphics cards in the earliest RMVideo workstations lacked multi-monitor support. Now that newer cards are available with multiple DVI-D, VGA, and/or DisplayPort outputs, it is possible to drive a second display to mirror what's being presented to the subject on the main monitor.
Xin Huang's lab at the University of Wisconsin has succeeded in doing so. Here is the procedure they use to drive a second display (many thanks to Dr. Huang and Bryce Arseneau):
Before powering up the Linux workstation, make sure only ONE monitor is plugged into the NVidia graphics card in the machine. Start the computer and verify that RMVideo launches without error. If RMVideo crashes, double-check that only one monitor is connected to the workstation, and ensure that the current video mode (resolution and refresh rate) satisfies RMVideo's minimum requirements (1024x768, 60Hz or greater). Then relaunch RMVideo.
If RMVideo has launched successfully, connect the second monitor into an available output on the video card.
Click on the "start menu" button in the bottom left corner and find the NVidia X Server Settings application (usually under System Tools or Preferences; the location will vary with the particular Linux distribution you're using).
Click on the X Server Display Configuration tab in the navigation panel on the left-hand side of the NVidia app. Then click on the Detect Display button -- near the bottom in the right-hand panel. This should bring up the second monitor in the Layout section at the top of the right-hand panel. Use the mouse to drag the image of the newly detected monitor on top of the main monitor. This will mirror the display so that both monitors display the same output. At this point the second monitor should reflect what's shown on the main display.
Make sure that both monitors are set to the same video mode -- e.g., 1024x768 @ 100Hz; the second monitor may fail to work if it's video mode does not match that of the main monitor.
Launch Maestro. In the Idle mode control panel, click on the Video Display tab and ensure that the video mode is set to the desired resolution and refresh rate.
Note that RMVideo itself is not "aware" of the second monitor (that may be why RMVideo crashes if you start it with two monitors connected to the graphics card). The available video modes reported on Maestro's Video Display tab are those modes supported by the main monitor that satisfy minimum requirements. It is important that the second monitor support all of the video modes that you plan to use on the primary display; mirroring will likely fail if you select a video mode that the second monitor cannot handle.