The easiest way to concatenate video files is by FFmpeg.Conversions.FromSnippet.Concatenate:

string output = Path.ChangeExtension(Path.GetTempFileName(), ".mp4");
var conversion = await FFmpeg.Conversions.FromSnippet.Concatenate(output, Resources.MkvWithAudio, Resources.Mp4WithAudio);
await conversion.Start();

Concatenation works with almost unlimited number of files.

Concatenate snippet is one of the most complicated logic in whole library. Providing one generic method to join a lot of file types required few sacrifices. One of them is speed. Concatenation if done by filter_complex concat and all of input videos are scaled to the one with highest resolution. A few other options are set too like aspect, format or anullsrc.