Last week's Catalyst 15.7 WHQL driver from AMD unifies the driver structure between the R9 300 and R9 200 series. What this means is that it's now possible to put a Radeon R9 290X and a Radeon R9 390X card in CrossFire. The downside of course is that this lowers the performance of your R9 390X down to the 290X level, we'll have to wait for DX12 adoption to get more flexible multi-GPU support.
You may even create a 4-way CrossFire setup with one each of the four cards. You should even be able to combine R9 380 with R9 285. AMD allowed similar cross-generation pairings in the past, such as those between the R9 280 series and HD 7900 series, and between the R9 270 series and HD 7800 series. The company's newer GPUs feature XDMA CrossFire, which does away with bridge cables, for increased flexibility.