IVideoStream and IAudioStream allows split media but the easiest (but not most efficient) way is by FFmpeg.Conversions.FromSnippet.Split:

string output = Path.ChangeExtension(Path.GetTempFileName(), ".mp4");
IConversion conversion = await FFmpeg.Conversions.FromSnippet.Split(Resources.Mp4WithAudio, output, TimeSpan.FromSeconds(2), TimeSpan.FromSeconds(8));
IConversionResult result = await conversion.Start();

Helper splits all streams in media file and copies them (splitted) to output. In example output contains a media file with duration of 6 seconds contains both audio and video streams.