You can refer following links hope you get some help :-
http://www.iphonedevsdk.com/forum/iphone-sdk-development-advanced-discussion/77999-make-video-nsarray-uiimages.html
Using FFMPEG library with iPhone SDK for video encoding
Iphone SDK,Create a Video from UIImage
iOS Video Editing - Is it possible to merge (side by side not one after other) two video files into one using iOS 4 AVFoundation classes?