[FFmpeg-user] how to combine 2x stereo.wav to 1x stereo.wav while changing the panorama?

[LaHu]

Hi,

I have two different .wav files with different durations.
File a: english narration on channel 1, effects on channel 2
File b: french narration on channel 1, original sound on channel 2

The result should be one stereo.wav file:
english narration and effects mixed together to the left channel
french narration and original sound mixed together to the right channel

I only know how to do one part:
ffmpeg -i engl_fx.wav -af pan=stereo:c0=c0+c1 engl.wav

How could I do all in one step, regardless of the duration of the files?

Thanks for suggestions!
LaHu