this is an example of multiple exposures done in photoshop: four regular photos layered into one file. the blending mode that corresponds most closely to in-camera multiple exposures is the screen blending mode, but for that to look right you have to underexpose each individual shot accordingly, just like when doing it in-camera. as the source images were exposed normally in this case, i added an exposure adjustment layer set to -3 for each individual image layer (in theory should be 1/4 the exposure = -2, as I have four layers set to screen blending mode). finally, i increased the contrast with a curves adjustment layer (just like in-camera multiple exposures decrease contrast, so do multiple exposures in photoshop).

i also tried to mimic the in-camera multiple exposure i posted earlier, but couldn't make it look as nice with source images taken from similar angles. i was not able to create the same painterly softness in photoshop as i got in-camera. The blending and gradation of the colors just seem softer and with a finer gradation in the in-camera version, while the photoshop version was full of "spikes" in the histogram. this sharper impression worked better in this image, due to the nature of the subject (less organic and more man made structures).