var command = "ffmpeg -i "+ input1+ " -itsoffset 40 -i " + input2 +" -filter_complex amerge -c:a libmp3lame -q:a 4 "+ output;

        exec(command, function (error, stdout, stderr) {
            if (stdout) console.log(stdout);
            if (stderr) console.log(stderr);

            if (error) {
                console.log('exec error: ' + error);
                response.statusCode = 404;
                response.end();

            } else {
                // Do something
            }

        });

ffmpeg -i 0.mp3 -i 1.mp3 -i 2.mp3 -filter_complex
"aevalsrc=0:d=10[s1];
aevalsrc=0:d=20[s2];
[s1][1:a]concat=n=2:v=0:a=1[ac1];
[s2][2:a]concat=n=2:v=0:a=1[ac2];
[0:a][ac1][ac2]amix=3[aout]" -map [aout] out.mp3