NOTE:- Merge Videos Side By Side WITHOUT Losing Video Quality
I think that is a Very Very Important Question, After a lot of search
in fact, AVAssetExportSession is for simple needs, and it is too simple for your situation.
You must use AVAssetWriter.
You add AVAssetWriterInput to your AVAssetWriter.
You can configure trasnform of the AVAssetWriterInput using its transform property.
Then, you feed your AVAssetWriterInput with CMSampleBuffer (each images buffer) using append calls.
See full Apple documentation for detailed example: https://developer.apple.com/library/archive/documentation/AudioVideo/Conceptual/AVFoundationPG/Articles/05_Export.html#//apple_ref/doc/uid/TP40010188-CH9-SW2