[FFmpeg-user] Joining two videos

[Shiwani Agrawal]

Hi all , 
I have two files one is having both audio video stream(180 secs) and other is having only video(30 secs) . The output required is such that the for initial part there is only the first video on whole screen then after 20 secs the second video should come and the first video overlays on top left corner of it then after completion of the second video first one should again come whole screen till the end. 
Please tell the filter or any other option with the help of which I can do this in ffmpeg .
Thanks ,Shiwani