There's an easier way of doing that in Photoshop and with probably more convincing results by just using one image that has been either exposed for the sky or for the ground.
Say for example, the ground is well exposed but the sky is slightly over exposed but not blown out. You can make a levels adjustment and pull in the highlights....it will make the ground appear very under-exposed but that's fine. Then add a mask layer and fill it in black so that the levels adjustment disappears and the image is back to how it was. The just take a white paintbrush and paint the sky back in and voila!....the ground and the sky will be perfectly exposed
Hope that makes sense! It's a lot easier to do than to explain it
Nikon D300 / Sigma 10-20mm / Nikkor 17-55 f2.8 / Nikkor 18-200mm VR / Nikkor 50mm f1.8 / SB600 Speedlight
Mark Amy Photography
My Squidoo lens - Images of China