I'm using Arch Linux, but had this issue with other distributions as well: on HP Pavilion G6 laptop with Intel and Radeon graphic card.
When I modify the xorg.conf file to enable dualhead (such as: VGA1, leftOf, LVDS) using the Intel driver it works fine, except for LVDS ("laptop monitor") that had brightness set to 0 and was blank. Using media keys I could increase it and it was fine, but when it woke up from the screensaver it was blank again.
Is this a graphic driver issue or an ACPI issue? Can I disable it somehow, or change the default value of the brightness? I've tried video=SVIDEO-1:d kernel parameters and some other hints from Arch Wiki, but it didn't work for me.
This is irrelevant but still weird: when I make VGA1 (external monitor) primary and LVDS rightOf it, after boot I have to modify xorg.conf and change VGA1 to VGA2 for the dualhead to work. After restart it's VGA1 again and so on...