The Radeon HD6450 seems to be able to drive two displays at once. You should probably keep the card connected the way you have right now through your receiver, and connect a second cable from your video card's other output - probably the DVI one - directly to your other display. No use trying to use onboard video together with your video card - use the two outputs from your video card.
I am more familiar with dual displays using NVidia cards than Ati, but I will tell you that setting up a dual display should be fairly easy. Usually, whenever I play something in 3D, the software "knows" to switch into 3D mode without me telling it to.
When I was first setting up the system, it seemed the driver software recognized my 3D display better when it was the only one connected. Then, after I had 3D working, I connected my 2D display, and the system recognized it fine, adding it as a second display. Then I was able to make the 2D display my primary display (but you don't even ever have to do that, anyway.)
Others will probably add more specific advise for Radeon cards.