So, how do you create an image like this? Firstly, not all DSLRs or mirrorless cameras have a multiple exposure mode. I use a Canon 5D Mark iii, which has a setting for multiple exposures. While it is easy to set the camera to the appropriate settings and then take two or more images that are combined, the difficulty is in choosing the images, framing and positioning these shots so that they blend well into one image.
The first exposure is considered the base layer on which your second image will blend into. In the shot above, I really wanted the Opera House to fit entirely in the bulk of the Sirius Building. With that in mind, I tried to position the buildings when pressing the shutter in a way that would create this effect. I kept the exposure the same for each of the images and was rewarded with the above multiple exposure.
Like with most photography, the best way to understand double or multiple exposure is to experiment. While there are no steadfast rules, it is useful to remember that darker subjects blend more easily than light ones. You don't want to end up with a bunch of blown out highlights. Sometimes it is easier to shoot the darker scene first, then try a lighter scene on top. Of course, there are exceptions. If you are trying to create a silhouette of a person with its head or body filled with details from another exposure, shooting a dark silhouette first that has lightness around it, will allow the elements of your second exposure to fill in the void of the silhouette.
There are many interesting kinds of multiple exposures. I love clouds and ocean and am always trying to get the perfect double exposure in camera. Sometimes I use a slow shutter speed to capture the ocean in motion, and then a faster one to capture the clouds. I almost always use a tripod, and for my image below, entitled "Impending", I moved the camera ever so slightly downwards with the second image so that the end result looks like the clouds are just hovering above the water.