Multiple exposure is the superimposition of two or more individual exposures to create a single photograph. The exposure values may or may not be identical to each other. Unless you have a camera that was designed to do this, you're most likely going to have to use photoshop to put your pictures together.
You must limit it to 2 images, and blending only, using the complete images, e.g no masks.
You are not allowed to mask, selectively remove, etc. Basically you take one image put it on top and lower the opacity or change blend mode.
You can either do it incamera or use Photoshop. If possible, show us the single shots in the SBS.