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If I try to launch something with optirun under compiz, it works.

If I try to launch something with optirun under KDE or metacity, it gives me:

[  247.384077] [ERROR]Cannot access secondary GPU - error: [XORG] (EE) 

[  247.384117] [ERROR]Aborting because fallback start is disabled.

If it matters, I'm trying to launch Portal 2 with wine

I have:
Nvidia GeForce GT540M with optimus
Acer Aspire Timeline X
Intel core i5 and 3000 Integrated Graphics

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Have a try with primus, which has different options for different configurations:

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