I have two video cards, one running each of two identical monitors. My main card is an Nvidia GeForce 750 Ti, and the secondary is a Radeon HD5500. When I boot from live USB, the monitor attached to the HD5500 displays the desktop, and the other displays a gray screen with a flashing cursor (but doesn't do anything else). After installing, the desktop shows up on the GeForce, and the Radeon monitor is blank.
I followed this guide to attempt to install proprietary Nvidia drivers, but it didn't work. So I tried downloading the package straight from Nvidia. I think that worked, but I'm not sure (I'm completely new to this). I then tried to select a different driver for the Radeon from the list on the Additional Drivers settings tab. Now when I boot Ubuntu, the Radeon monitor gets no signal, and the GeForce monitor displays black.
What did I do wrong, and how do I fix it?
Ubuntu 14.04 dual boot with Windows 8.1 U1