Muliple exposure composite

A much easier way to accomplish the same thing is by using layer masks.

Create a photoshop document and create 2 layers. Put the "highlight" image in the bottom layer and the "lowlight" image in the top layer.

Select the highlight image. Create a selection that reflects the "grayscale" of the highlight image. Apply this selection as a layer mask to the lowlight layer. Use curves on the layer mask to adjust the blending of the two layers/images (curves used in this example).

Of course one can go back and make finetunings on the layer mask and end up with more or less the same as in the first example.