Multiple exposure..The same from InCamera and Photoshop??

Many people think it is just easier to superimpose in PS and will give the same result
Multiple exposures through the camera or superimpose in Photoshop are not the same.
When you make a double exposure in the camera best results are when working with a dark background.
When superimposing through Photoshop it is just the other way...The second image will be visible only through a light background.
Made a test both ways..
One is made in the Camera, the other one were two exposures superimposed in CS3.
However close, note the highlights..up to you which image was made directly in the camera.

How did you construct the PS version?

It depends on the layer blending mode you choose.
Normal, Dissolve, Darken, Multiply...

Two factors:

#1: Photoshop does not operate in a linear light colorspace by default, so multiple exposures won't look "right" until you address that. The easiest way in newer versions of PS is to switch to 32 bits/channel. That should switch you over to a linear profile. From there, use the "Linear Dodge (Add)" blend mode to combine your multiple exposures.

In older versions of Photoshop, you can use the "Screen" blend mode to approximate the look of exposure accumulation.

#2: The above method acts like a film with no highlight shoulder -- in other words, it acts like a digital camera. Film, however, has softer highlights (especially neg film). The result is that two highlights exposed on top of each other may look "blown out" with the above method, whereas on film there's a gentler transition to the highlights.

The solution is to Merge back to 8 or 16 bit, and use one of the Merge methods to approximate what you think your film would do. My preferred method is to use Highlight Compression and then apply some curves after that to boost the highlights and play with the midtones a little (Highlight Compression really needs an "amount" slider!).

PS! The Photoshopped version is on the left.