In my guest blog over at scottkelby.com I made reference to a situation where I was forced to take a quick shot and used two exposures from Camera Raw to create what I would have liked to get in-camera - if time had permitted. A few people have asked how I did that technique, so here's a relatively quick overview....
1. After setting the size and resolution in Camera Raw, I exposed for the sky, ignoring what it would do to the wall. I increased the exposure a little, pushed the Recovery slider all the way to try to deal with the areas of the sky that were over-exposed, and played with color temperature, Vibrance and Saturation.
2. After opening the image into Photoshop, I used the Open Recent menu to open the same file again. Since the file was in Raw, it opened again in ACR. This time I exposed for the wall, knowing that it would blow out the sky. I had to increase Exposure and lower Recovery to get the wall to look the way I wanted. Then I opened this 2nd version of the photo.
3. Using the Move tool with the Shift key held down, I dragged the version with the over-exposed sky onto the document with the "good" sky. This created a second layer (and by holding down Shift as I dragged, I'm assured that the 2 layers are perfectly aligned).
4. Since the sky is so overexposed, the first thing I typically try is the Blend If sliders - when this works, it's almost like magic, but some times it doesn't work quite so well and I go with plan b (see below). Double-click on the top layer to open the Layer Styles dialog and use the white slider under the "this layer" slider to drag to the left. This makes all the white areas transparent: in this case it makes the over-exposed sky transparent, letting the "good" sky show through. (I also held down Option/Alt to split the triangle to make a softer transition to the transparent area)
5. There were a few areas that didn't quite look as good as would have liked, so I added a Layer Mask to manually paint (with black) a few edges to hide them and let more of the good sky show through.
When the blend if sliders don't work very well, I usually use a Channel-based selection to make a layer mask:
1. I choose the channel that has the best contrast between the sky and the wall, in this case the Blue Channel and duplicate it.
2. Then I use the Apply Image command several times to make the sky area whiter and the wall area darker.
3. I make a large selection of all the areas inside the wall and fill with black to create the look of the mask that I wanted (it's still a channel at this point)
4. I Command/Ctrl click on the channel thumbnail to turn it into a selection and on the top layer add a layer mask (in the case I had to Invert the mask to get the desired effect, which was to mask the over-exposed show the layer below shows through)
By the way, I could have done this whole thing using Smart Objects from ACR, but that's another tutorial for another time...
Hope this helps demonstrate how you can combine 2 (or more) exposure from one Raw file.