Deactivate one graphicdevice in a SLI system

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http://askubuntu.com – As it is now, I have a Gaminglaptop (Dell XPS M1730, Build Dec 2008) with 2 GeForce 9800M GT with SLI activated, Ubuntu 12.04. When I use any graphicdriver that is aware of acceleration features the system crashes after a short period of time. After I removed and purged jockey my system runs stable but slowly, but I can use it for now (I have to choose recovery at startup and the resume boot, botting up normal results in a colorflickering screen). But since the system is runs slowly, I would like to try to work with only one graphicdevice. First test device A, if that doesn't work test wit (HowTos)