A pair of R7-370 would use around 220W on average vs 300W for the R9-390X, not such a huge difference. For peak power, it would be 360W vs 470W, not that far apart either. You will need a somewhat substantial PSU either way.
However, using the R9-390X guarantees you at least twice the performance of an R7-370 under practically all circumstances with no chance of crossfire frame stuttering or the need to depend on game-specific crossfire support.
If you want hassle-free gaming, the R9-390(X) would generally be a better option than R7-370 x2.
Edit: of course, if power efficiency is your main concern, the GTX970 would be an even better option.