(In reply to comment #49)
> 
> Is the BusID line in the intel device section not needed? What is needed for
> X to automatically detect multiple GPUs properly and show some video output
> from the correct card?

You'll need to use vga-switcheroo to select the card you want to use.  Then you
should be able to start X without any config and it will start on the one
selected by vga-switcheroo.  Having two device sections as your config does
will force X to try and start on both devices.