There's an app called Machete that can do this, but it has obvious constraints - can only cut/paste at keyframe boundaries, video/audio streams must be compatib

There's an app called Machete that can do this, but it has obvious constraints - can only cut/paste at keyframe boundaries, video/audio streams must be compatible. In practice it's useful for editing one source video, but not for combining separate vids from different sources. Not free but cheap.