I created a tool in Processing that lets me mash up two (or more) videos together. The sketch allows the user to change the interval between the videos and their transparency values, and also keeps track of the changes so the same set of transitions can be played again later.
Here are a few samples:
[vimeo http://vimeo.com/89869267 w=640]
[vimeo http://vimeo.com/89883436 w=640]