I can confirm with my build, this works flawlessly. After numerous searches across the internet this is how to do it. This is how I did it:
First of all, I'm using Clover and install Yosemite using Clover.
1) First, you need to go to your BIOS and select iGFX as your display output.
2) When you boot up to the Clover bootloader, go to Options > Graphics Injector Menu and then change the FBName to: Hawaii (this is for the R9 290X, I believe if you know the CPU name, type in the name in the input).
3) Disconnect the second monitor if you have it connected, this caused me to crash at start-up.
4) When Yosemite is booted up, go up to the Apple Menu and then "About this Mac".
5) Go to "Display" and then reconnect the second monitor. (I've used DVI on the first monitor and on this step, I reconnected the second DVI port to another monitor).
6) Hey presto, this should have your dual screens.
Here is my build that works perfectly on Mac OSX Yosemite 10.10:
CPU: Intel Core i7 4770k
GPU: XFX R9 290X / 4GB RAM DDR5
MOBO: Gigabyte GA-Z87X-D3H
(Watercooled as well ;D)