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.