It turns out that the latest AMD graphics drivers, namely Catalyst 15.7, unveiled on July 8, 2015, allow gamers and hardware enthusiasts to create CrossFire setups which consist of one Radeon R9 300 GPU and another Radeon R9 200 GPU.
This is the result of the unification of the driver for both GPU generations as well as the fact that the R9 300 line is more or less the same R9 200 line, just rebranded. Now you can pair a Radeon R9 390X card with a Radeon R9 290X or a Radeon R9 390 card with a Radeon R9 390 card. The new drivers also allow the creation of a 4-way CrossFire setup with one each of the four cards.
The new CrossFire setup follows previous such setups – a few years ago AMD also allowed the creation of CrossFire setups, which included R9 280 and HD 7900 video cards as well as R9 270 cards and HD 7800 cards. In addition the new GPU generations do not need bridge cables for increased flexibility.
Source: Techpowerup.com