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

Output:
[Parsed_drawtext_1 @ 0000000003202c40] Both text and text file provided. Please provide only one
[AVFilterGraph @ 0000000003200c00] Error initializing filter 'drawtext' with args 'fontfile=C:ffmpeg-20170921-183fd30-win64-staticbinFreeSerif.ttf: textTest Text: x=100: y=50: fontsize=24:'
Error initializing complex filters.
Invalid argument