Can I Crossfire r9 270X and r9 390?

Solution
No, the cards need to be based on the same GPU, you could crossfire a 270 and a 270X, at which point they would run at the speed of the slower card.

The 390 far outperforms the 270X, so even if you could, it would be a negative impact on performance.
No, the cards need to be based on the same GPU, you could crossfire a 270 and a 270X, at which point they would run at the speed of the slower card.

The 390 far outperforms the 270X, so even if you could, it would be a negative impact on performance.
 
Solution
No you cant crossfire 2 different GPU chips like that.
Crossfiring will provide more processing power and memory for optimized games.

In practice though, between lack of support in games, drivers, and other factors it is more effective as a whole to just have the best single graphics card you can afford