Hello,
Short answer: no
Longer answer: the R9 280x and 6850 can not crossfire as they are based off of different GPUs.
Crossfire typically works with two (or more) of the same video card, or video cards with similar GPUs inside. For example, a 6850 will crossfire with a 6870, but will behave as a 6850 + 6850.
Cheers,