Use
ffmpeg -i in0.mp4 -i in1.mp4
-filter_complex
"[0]fade=t=out:st=0:d=1:alpha=1,setpts=PTS-STARTPTS[va0];
[1]fade=t=in:st=0:d=1:alpha=1,setpts=PTS-STARTPTS[va1];
[va0][va1]overlay[outv];
[0][1]amix[outa]" -map [outv] -map "[outa]" -crf 10 out.mp4