>ffmpeg -f gdigrab -framerate 30 -i title="" -f gdigrab -framerate 30 -i title="" -c:v h264_nvenc -filter_complex hstack=inputs=2 2.mp4
ffmpeg version 4.4-full_build-www.gyan.dev Copyright (c) 2000-2021 the FFmpeg developers
  built with gcc 10.2.0 (Rev6, Built by MSYS2 project)
  configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-lzma --enable-libsnappy --enable-zlib --enable-librist --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-libbluray --enable-libcaca --enable-sdl2 --enable-libdav1d --enable-libzvbi --enable-librav1e --enable-libsvtav1 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-libaom --enable-libopenjpeg --enable-libvpx --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-ffnvcodec --enable-nvdec --enable-nvenc --enable-d3d11va --enable-dxva2 --enable-libmfx --enable-libglslang --enable-vulkan --enable-opencl --enable-libcdio --enable-libgme --enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libilbc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --enable-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint
  libavutil      56. 70.100 / 56. 70.100
  libavcodec     58.134.100 / 58.134.100
  libavformat    58. 76.100 / 58. 76.100
  libavdevice    58. 13.100 / 58. 13.100
  libavfilter     7.110.100 /  7.110.100
  libswscale      5.  9.100 /  5.  9.100
  libswresample   3.  9.100 /  3.  9.100
  libpostproc    55.  9.100 / 55.  9.100
[gdigrab @ 00000249105ef6c0] Found window , capturing 349x488x32 at (0,0)
Input #0, gdigrab, from 'title=':
  Duration: N/A, start: 1630274478.398846, bitrate: 163512 kb/s
  Stream #0:0: Video: bmp, bgra, 349x488, 163512 kb/s, 30 fps, 30 tbr, 1000k tbn, 1000k tbc
[gdigrab @ 0000024910c96440] Found window , capturing 862x500x32 at (0,0)
Input #1, gdigrab, from 'title=':
  Duration: N/A, start: 1630274478.670650, bitrate: 413772 kb/s
  Stream #1:0: Video: bmp, bgra, 862x500, 413772 kb/s, 30 fps, 30 tbr, 1000k tbn, 1000k tbc
File '2.mp4' already exists. Overwrite? [y/N] y
Stream mapping:
  Stream #0:0 (bmp) -> hstack:input0
  Stream #1:0 (bmp) -> hstack:input1
  hstack -> Stream #0:0 (h264_nvenc)
Press [q] to stop, [?] for help
[Parsed_hstack_0 @ 000002491166fd00] Input 1 height 500 does not match input 0 height 488.
[Parsed_hstack_0 @ 000002491166fd00] Failed to configure output pad on Parsed_hstack_0
Error reinitializing filters!
Failed to inject frame into filter network: Invalid argument
Error while processing the decoded data for stream #1:0
[gdigrab @ 00000249105ef6c0] Thread message queue blocking; consider raising the thread_queue_size option (current value: 8)
[gdigrab @ 0000024910c96440] Thread message queue blocking; consider raising the thread_queue_size option (current value: 8)
Conversion failed!