If you look at pure performance, yes 2 mid range cards will beat 1 high end card.
However, I would still choose 1 high end card. I find 2 cards setups have too many drawbacks: they draw more power so you need a beefier PSU, they will generate more heat inside your case, you usually get more drivers problems and instability with 2 cards setups, etc...
It also depends what resolution and how many monitors you're using. If you only have 1 monitor with 1080p resolution, 1 card is plenty enough especially a R9 280x.