How to lay one video over another
It is not clear to me what you mean by laying "one video over another." This could mean you want to show both simultaneously in the same frame. One way to do this is using a split screen. Another is to superimpose each over the other then reduce the opacity so you see both images. Yet another is to superimpose each over the other but use a mask on the top layer so only portions of the bottom layer are exposed.

Summary of possible overlays with links to examples.

1) Split Screen
https://vimeo.com/22564317

2) Double Exposure
https://vimeo.com/42863351

3) Compositing
https://vimeo.com/44747735

You will need an NLE that is capable of rendering multiple video tracks at once. Sony Vegas Movie Studio would allow up to 4 video simultaneously, while Vegas Pro will allow as many as your computer can handle (on my 3 year old quad core this can be up to 9 SD tracks or 5 HD tracks). Other NLE video editors to check out would be Avid, Adobe Premiere, and FCP.

Once you know which one of the effects I listed above fits your situation, you will be able to find tutorials on YouTube specific to the NLE you are using. For instance, here is a tutorial I found on YouTube by searching for "how to use split screen on vegas movie studio".

https://www.youtube.com/watch?v=HnsWKDXIxJA