On the Mac, I just use quicktime Pro to combine two or more separate videos into a single one. I have not tried doing this on Linux yet but figured that it won't be long before I find a reason to maximizing the power of all my computers in processing some home videos.
I have featured several tools for video editing before and I am adding another one to the collection. For simple tasks such as combining videos, try Avidemux, if the other tools are over-kill for your needs. Here's a simple tutorial from Ubuntu Essentials to guide you in using Avidemux.