CrossFire isn't supported on Linux so right now you can't utilize both of these GPUs. However, as we exclusively shared last month, CrossFire is coming to Linux and it will be here later this quarter. When that time comes, you'll be able to benefit from these two RV770 cores on Linux. We'll also be delivering CrossFire and Radeon HD 4870 X2 benchmarks at that time.
If you're just concerned about basic mode-setting support, the Radeon HD 4870 X2 graphics card should work with the xf86-video-ati or AtomBIOS-ized xf86-video-radeonhd drivers after adding in the new PCI ID. However, don't look to use CrossFire yet with any open-source driver let alone any 3D support right now until the R600/RV770 programming documentation and sample source-code gets released.