My Command:
ffmpeg -f dshow -video_size 1920x1080 -framerate 30 -vcodec mjpeg -i video="HD Pro Webcam C920" -f dshow -video_size 1920x1080 -framerate 30 -vcodec mjpeg -i video="HD USB Camera" -preset ultrafast -threads 0 -filter_complex "[0:v][1:v]hstack" drawtext="fontfile=C:\ffmpeg-20170921-183fd30-win64-static\bin\FreeSerif.ttf: text'Test Text': x=100: y=50: fontsize=24:" -c:v h264_qsv dualwtext.ts

Error:
[NULL @ 00000000074cdde0] Unable to find a suitable output format for 'drawtext=fontfile=C:\ffmpeg-20170921-183fd30-win64-static\bin\FreeSerif.ttf: text'Test Text': x=100: y=50: fontsize=24:'
drawtext=fontfile=C:\ffmpeg-20170921-183fd30-win64-static\bin\FreeSerif.ttf: text'Test Text': x=100: y=50: fontsize=24:: Invalid argument