ffmpeg -i foo.mp4 -filter_complex "fade=d=0.5, reverse, fade=d=0.5, reverse" output.mp4

can be used to fade in and out foo.mp4 video. (we do not care about audio). According to https://video.stackexchange.com/questions/19867/how-to-fade-in-out-a-video-audio-clip-with-unknown-duration

It's good but only works in the simple situation of 1 input video, and 1 output. Now, how can I apply the fade in and out effect in the following more complex situation? I'm trying to concat a.jpg (a picture) with bar.mp4. I want only the bar.mp4 portion to fade in and out.

ffmpeg -loop 1 -t 2 -framerate 1 -i a.jpg  -f lavfi -t 2 -i anullsrc -r 24 -i bar.mp4 -filter_complex "[0][1][2:v][2:a] concat=n=2:v=1:a=1 [vpre][a];[vpre]fps=24[v]" -map "[v]" -map "[a]" out.mp4 -y

Of course, I could first create a temporary temp.mp4 from bar.mp4 by running the first command, then input temp.mp4 in my second command. This involves an extra step and extra encoding. Could anyone help fix the commands or suggest something even better?