$ ffmpeg -f lavfi -i sine -f lavfi -i sine -af aresample=ocl=stereo -map 0:a:0 -map 0:a:1 -f null -

Input #0, lavfi, from 'sine':
  Duration: N/A, start: 0.000000, bitrate: 705 kb/s
    Stream #0:0: Audio: pcm_s16le, 44100 Hz, mono, s16, 705 kb/s
Input #1, lavfi, from 'sine':
  Duration: N/A, start: 0.000000, bitrate: 705 kb/s
    Stream #1:0: Audio: pcm_s16le, 44100 Hz, mono, s16, 705 kb/s
Stream map '0:a:1' matches no streams.
To ignore this, add a trailing '?' to the map.