Don't do this. Bad idea. FCPX gets terribly confused.
You can't specify which cards FCPX uses and you could end up in a right mess.
FCPX Help said:Render/Share GPU
- Render/Share GPU: Use this pop-up menu to choose which graphics processing unit (GPU) to use in Final Cut Pro when rendering or sharing. This choice affects only rendering and sharing; real-time effects use the default GPU.