FFMPEG merge audio tracks into one and encode using NVENC - ffmpeg

Question

I often shoot films with several audio inputs, resulting in video files with multiple audio tracks supposed to be played all together at the same time.
I usually go through editing those files and there I do whatever I want with those files, but sometimes I would also like to just send the files right away online without editing, in which case I would enjoy FFMPEG's fast & simple & quality encoding.
But here's the catch: most online video streaming services don't support multiple audio tracks, so I have to merge them into one so we can hear everything.
I also want to upscale the video (it's a little trick for the streaming service to trigger its higher quality encoding).
And finally, since it's just an encoding meant to just be shared on a streaming service, I prefer a fast & light encoding over quality, which HEVC NVENC is good for.
So far I've tried to use the amix advanced filter and I try to use the Lanczos filter for upscaling which seems to give a better result in my case.
The input file is quite simple:
Stream 0:0 : video track
Stream 0:1 : main audio recording
Stream 0:2 : secondary audio recording
The audio tracks are at the correct volume and duration and position in time, so the only thing I need is really just to turn them into one track
ffmpeg -i "ow_raw.mp4" -filter_complex "[0:1][0:2]amix=inputs=2[a]" -map "0:0" -map "[a]" -c:v hevc_nvenc -preset fast -level 4.1 -pix_fmt yuv420p -vf scale=2560:1:flags=lanczos "ow_share.mkv" -y
But it doesn't work:
ffmpeg version N-94905-g8efc9fcc56 Copyright (c) 2000-2019 the FFmpeg developers
built with gcc 9.1.1 (GCC) 20190807
configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-fontconfig --enable-gnutls --enable-iconv --enable-libass --enable-libdav1d --enable-libbluray --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-libvorbis --enable-libvo-amrwbenc --enable-libmysofa --enable-libspeex --enable-libxvid --enable-libaom --enable-libmfx --enable-ffnvcodec --enable-cuvid --enable-d3d11va --enable-nvenc --enable-nvdec --enable-dxva2 --enable-avisynth --enable-libopenmpt --enable-amf
libavutil 56. 35.100 / 56. 35.100
libavcodec 58. 56.101 / 58. 56.101
libavformat 58. 32.104 / 58. 32.104
libavdevice 58. 9.100 / 58. 9.100
libavfilter 7. 58.102 / 7. 58.102
libswscale 5. 6.100 / 5. 6.100
libswresample 3. 6.100 / 3. 6.100
libpostproc 55. 6.100 / 55. 6.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'ow_raw.mp4':
Metadata:
major_brand : mp42
minor_version : 0
compatible_brands: isommp42
creation_time : 2019-11-02T16:43:32.000000Z
date : 2019
Duration: 00:15:49.79, start: 0.000000, bitrate: 30194 kb/s
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, smpte170m/smpte170m/bt470m), 1920x1080 [SAR 1:1 DAR 16:9], 29805 kb/s, 60 fps, 60 tbr, 90k tbn, 120 tbc (default)
Metadata:
creation_time : 2019-11-02T16:43:32.000000Z
handler_name : VideoHandle
Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 196 kb/s (default)
Metadata:
creation_time : 2019-11-02T16:43:32.000000Z
handler_name : SoundHandle
Stream #0:2(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 184 kb/s (default)
Metadata:
creation_time : 2019-11-02T16:43:32.000000Z
handler_name : SoundHandle
Stream mapping:
Stream #0:1 (aac) -> amix:input0 (graph 0)
Stream #0:2 (aac) -> amix:input1 (graph 0)
Stream #0:0 -> #0:0 (h264 (native) -> hevc (hevc_nvenc))
amix (graph 0) -> Stream #0:1 (libvorbis)
Press [q] to stop, [?] for help
[hevc_nvenc # 000002287e34a040] InitializeEncoder failed: invalid param (8)
Error initializing output stream 0:0 -- Error while opening encoder for output stream #0:0 - maybe incorrect parameters such as bit_rate, rate, width or height
Conversion failed!