Why does ffmpeg stop randomly in the middle of a process?

ffmpeg feels like its taking a long time. I then look at my output file and i see it stops between 6 and 8mbs. A fully encoded file is about 14mb. Why does ffmpeg stop? My code locks up on StandardOutput.ReadToEnd();. I had to kill the process (after seeing it not move for more then 10 seconds when i see it update every second previously) then i get the results of stdout and err. stdout is “” stderr is below.

Merge two video clips into one, placing them next to each other

I have two video clips. Both are 640×480 and last 10 minutes. One contains background audio, the other one a singing actor. I would like to create a single 10 minute video clip measuring 1280×480 (in other words, I want to place the videos next to each other and play them simultaneously, mixing audio from both clips). I’ve tried trying to figure out how to do this with ffmpeg/avidemux, but so far I came up empty. They all refer to concatenating when I search for merging.