Would need more information on the power supply. Need you to tell us the model number/brand or if you know what what the 12 volt rail(s) wattage, amperage, and all the connectors that are on those rails.
However I do not recommend cross firing. I myself own two Radeon HD 7950's in crossfire. Trust when I say the performance gain, in the games that support it (and that isn't a lot), is only about 40%. On top of that, the draw backs have been frame stuttering (like that random stutter you get when a open world game loads the next map chunk), terribly inconsistent slow frame times (makes 60fps feel like a weird 30), higher temperatures cross the board inside my PC, and not to mention the awful and mostly lack of optimized drivers for it. Drivers usually not coming out for newer titles until months after they're release. Making one of your cards completely useless. On top of that the VRAM dosen't stack either, so even if you may have the horsepower to run something that require 3GB of VRAM or more- your're out of luck.
The only reason to crossfire on any card that isn't a flagship, is if you can get a 2nd one for really cheap. Like you find a 270X for 60$ on ebay/craiglist that works. To the point of where it becomes a "why not" scenario.
The R9 285 all the way. If you could get a 4GB variant that'd be the best situation to be in as it'd let you run games (with reduced settings) internaly at 4k, making everything look supper smooth. Once you go 4k you don't go back.
Direct X 12 will not support combined VRAM pooling despite some miss leading articles. It will let devs use multiple GPU's separately (and their respective VRAM) instead of making them copy things across both, but it does not pool it. One card can't access the others.
99% of the time, I edit for type-o's and grammar. I'll note it if that's not the case.
I write near essays for most my responses, and then try to condense as best I can to the introduction of one. Less is more. Let me know if I post to much.
I do a lot of spacing for readability. Let me know if that makes my posts seem to big.