Use your favourite digital image retrieval program to convert the dark and light frames to 8 bit TIFFs. This technique will work on JPGs as well, but not on RAW files. If you've imported your files into Photoshop in 16 bit mode, reduce them to 8 bit. Update: You can work in 16 bit mode in Photoshop CS, but the combined file size will get very large.
Put both images onscreen at the same time. Select the dark frame and press CTRL-A to select the whole thing. Press CTRL-C to copy it. Close that picture, as you no longer need it.
Select the light frame and press CTRL-V to paste the dark frame into it.
If you look at the Layers palette (Figure #1) you will now see your light image as Background Layer and your pasted dark image as Layer 1.
2. The Layer Mask
This is the easiest manual technique. Starting with having done the pasting of the dark image on the light one, add a Layer Mask. This is done by clicking on the second icon on the lower left of the Layers palette. You will now see a white rectangle next to the image on the Layer 1 layer. (See Figure #2).
Click on the background layer on the palette and the press CTRL-A to select the whole image. Press CTRL-C, copying it to the clipboard. Now hold down the ALT key and click on the white mask rectangle on the Layer 1 palette.
The whole image will now turn white. Next, press CTRL-V to paste the contents of the clipboard onto the white mask. You will now see a B&W mask image. With the B&W mask displayed go to Filter / Blur / Gaussian Blur and set the Radius to about 40 pixels. Click on the Background Layer and you're done.
Oh yes. With this technique you may want to select the background layer and add an appropriate curve to brighten up the dark area a bit prior to flattening the layers.
See Digital Blending