Here’s a new feature of Photoshop CS6 for photographers who remember the chemical darkroom. Go to the adjustments and add a Gradient Map adjustment layer.
Click on the down arrow next to the gradient and then on the arrow on the right to display the list of gradient types. Click on Photographic Toning.
Click on the Options arrow again and select Large List to see that the split-tones are based on traditional chemical processes. Click one to apply.