You would change a setting under CCC to not use ATI scaling. I remember seeing it but didnt play with it.
Second you will have two 1440x900 displays with a lot of black border around your 24" monitor.
When I tried Eyefinity with different sized monitors the displays defaulted to the lowest resolution. I dont know if they have changed that or not.
Im not sure of your goal. You would use eyefinity to span a single image across all monitors - think Google Earht across several monitors (I showed several people that and they were stunned). You could span the desktop across both monitors in which case you get native resolution on all monitors but your limited to one display for your image.