I am trying to concatenate two mpg files, both the source files play on any video player.

File 1: bleh.mpg, 1.5mb
File 2: blah.mpg, 1.4mb

First I do:

cat blah.mpg bleh.mpg > out.mpg

Then I repair the out.mpg like this:

ffmpeg -i out.mpg -acodec copy -vcodec copy final.mpg

The file final.mpg seems broken, it is 2.9 mb in size. it will not open with any video payer.
How can I debug this process? MPlayer gives me this error when I try to open final.mpg:

GstMpegAudioParse: No valid frames before end of stream