2-Channel Picture-In-Picture Video Generator

[gekoch]

Hy

I need a way to see two video signals on one video feed. So I got two cameras one is the main and the other one should be overlayed at the right lower corner (PIP).
I currently own this product here:
http://www.securitycamera2000.com/products/4-CH-Camera-Video-Quad-Processor-Splitter-with-VGA-Video-Output.html

It works great however I just need two inputs and it needs to be as small and light as possible. How hard is it to build one DIY with current available hardware like
Raspberry Pi etc.? Has someone experience in this kind of project?

Video-Signal: Pal / 25fps

Andy