Dual graphics cards in Mint & Ubuntu

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http://unix.stackexchange.com – I have a triple boot machine with Windows 7, Mint 13 (with XFCE) and Ubuntu 12. Only under Windows 7 does my second screen attached to a second graphics card come on. Under both Mint and Ubuntu, the second screen is blank and under my display properties I only seen on screen (the second screen enters power saving mode because it is receiving no signal). They both show up under lshw (as well as the on-board GPU on my motherboard but I don't wish to use this). Do I need to 'tweak' something specifically in Mint or Ubuntu to enable a second GPU? *-pci:0 description: PCI bridge produc (HowTos)