combine mp4 files

I have read masses of items on the web re this but failed to find a satisfactory answer. I want a way to QUICKLY combine two fairly large mp4 files - about 1.8GB each and in identical format straight off a video camera. At present I use mpeg Streamclip which is great and does the job in about a minute or so. However it looks as if it will not be made 64bit and will therefore shortly die. Using 'cat' script technique in terminal works fine with .mts files but with mp4 seems to have the (known) problem of combining the files but Quicktime etc only sees the first file due to some header issue which I (as a non expert) don't fully understand - and the solutions to that are more than a bit over my head. I know I can just add the files to Quicktime (or other software apps) and EXPORT but that is slow - for my purpose speed is important. Any ideas out there??