6850 is an old generation 40nm Barts architecture where as 270X 28nm Curacao XT architecture and you cant crossfire them. You can crossfire from same generation and same series cards like 6850 can be crossfired with 6870 and 6850, 6950 can be crossfired with 6950 and 6970, 7850 with 7850 and 7870 and so on. But you cant crossfire a different generation, series card . Hope you got it