ffmpeg (two audio stream + 1 video stream) -vf/-af/-filter and -filter_complex cannot be used together for the same stream

I'm trying to capture the screen video and two audio stream, one from mic, and the other one from analog monitor.

 ffmpeg -video_size "$__screen_size__" \ -framerate 50 \ -thread_queue_size 512 -f x11grab -i :0.0+0,0 \ -thread_queue_size 512 -f pulse -i "$__input_mic__" \ -thread_queue_size 512 -f pulse -i "$__output_monitor__" \ -filter_complex amerge \ -ac 2 \ -vcodec libx264rgb -crf 0 -preset:v ultrafast \ -acodec pcm_s16le \ -af aresample=async=1:first_pts=0 \ -async 1 \ -y \ "$__output__" 

The problem is ffmpeg complains about using two filter simultaneously for one stream:

Filtergraph 'aresample=async=1:first_pts=0' was specified through the -vf/-af/-filter option for output stream 0:0, which is fed from a complex filtergraph. -vf/-af/-filter and -filter_complex cannot be used together for the same stream. 

The script below (only one audio stream form mic) however works perfectly fine, when I don't have to merge audio streams

 ffmpeg -video_size "$__screen_size__" \ -framerate 50 \ -thread_queue_size 512 -f x11grab -i :0.0+0,0 \ -thread_queue_size 512 -f pulse -i "$__input_mic__" \ -vcodec libx264rgb -crf 0 -preset:v ultrafast \ -acodec pcm_s16le \ -af aresample=async=1:first_pts=0 \ -async 1 \ -y \ "$__output__" 

basically removing:

 -thread_queue_size 512 -f pulse -i "$__output_monitor__" \ -filter_complex amerge \ -ac 2 \ 

If I remove -af aresample=async=1:first_pts=0 and keep the above then the audio and video won't be synchronized.

Basically I want to merge audio streams without introducing any latency.