How to replace fl/fr 5.1 channels with l/r from another audio track

[EnergoStalin]

I want to mux channels to have full 6 channel experience while watching initial D. Sadly voiceover is only 2 channels. So i created this fairly simple ffmpeg command.

ffmpeg \
  -i Initial\ D\ -\ 1x01\ -\ The\ Ultimate\ Tofu\ Store\ Drift\ \ \ \[DarkDream\].mkv \
  -i Initial\ D\ -\ First\ Stage_\[GitS\]/Initial\ D\ -\ 1x01\ -\ The\ Ultimate\ Tofu\ Store\ Drift\ \ \ \[DarkDream\]_\[GitS\].mka \
  -filter_complex "
    [1:a:0]aresample=48000,channelsplit=channel_layout=stereo[l][r];
    [0:a:1]channelsplit=channel_layout=5.1[fl][fr][c][lf][rl][rr];
    [l][r][c][lf][rl][rr]amerge=inputs=6[ao]
  " \
  -map "[fl]" -f s16le /dev/null \
  -map "[fr]" -f s16le /dev/null \
  -c:v copy -map "0:v:0" -map "[ao]" -y out.mkv

Is there's any way to make it work with mpv --lavfi-complex option? Initially i tried it that way but it keeps insisting on mapping front_left and front_right channels. That makes me wondering maybe i choose wrong approach from the start or it was not possible at all?

The benefit of doing so is doubtable but i feel like writing some filters today.