I know I'm wasting my time trying to help you here but why are you even thinking about crossfiring two 270xs? You should have just bought a single better card and used that. All you do by adding in a second video card is get a slight increase in performance while doubling your heat output and consuming twice the power.
You would also need to replace your power supply as the one you put in (again, against our advice of not getting a cheap power supply) will not be nearly enough for two cards and will need to be replaced with a QUALITY unit of 700w+.
If you need more video card power, then you should sell the 270x and buy a better, single card - There is no reason to crossfire two lower end cards where there is a better single card available.
And for when you ignore all this and still go ahead and do it, there are loads of guides online that explain how to set it up, you don't need someone to repost all the information here for you.
That said, I know you won't listen to this and I'll await your new thread that will appear in a couple of weeks either saying "Why is my crossfire crashing/not working" or "Why did my PSU blow" when you are either trying to run two 270xs on your current PSU or after buying a junk "700w" unit.
