Mpeg Streamclip (free) will do it.
When you go to open select both the files by holding the CMD key. To say what order you would like them in you'll want to name the first one "1" and the second "2", in Finder just so the program is sure. Probably a simpler way, but thats how I have always done it.
Oh and install Perian (free), which basically increases the amount of file formats Mpeg Streamclip can work with.