crossfire Amd a10 6800k + r7 240 2gb ddr5

Solution
Given the age of the A10 6800k, finding a compatible GPU to pair with it may be difficult depending on where you're at:

http://www.cpu-world.com/info/AMD/Recommended_graphics_cards_for_AMD_dual-graphics.html

Though it does offer performance boosts in games that support it:

http://www.tomshardware.com/reviews/dual-graphics-crossfire-benchmark,3583.html

If your budget allows though, you may still be better off getting a newer GPU (almost anything with GDDR5), rather than trying to find the perfect dual GPU match.

http://www.amd.com/en-us/innovations/software-technologies/dual-graphics

Geekwad

Admirable
Given the age of the A10 6800k, finding a compatible GPU to pair with it may be difficult depending on where you're at:

http://www.cpu-world.com/info/AMD/Recommended_graphics_cards_for_AMD_dual-graphics.html

Though it does offer performance boosts in games that support it:

http://www.tomshardware.com/reviews/dual-graphics-crossfire-benchmark,3583.html

If your budget allows though, you may still be better off getting a newer GPU (almost anything with GDDR5), rather than trying to find the perfect dual GPU match.

http://www.amd.com/en-us/innovations/software-technologies/dual-graphics
 
Solution
I'd prefer one card, a used card so I could get maximum performance per dollar spent. But you don't provide a lot of important information. Like what is your budget? Is buying a used card an option? What kind of power supply do you have? What games do you play? What kind of performance are you hoping to achieve?