Video Masking with FFMPEG

hello ,
any idea about how can we add one black-white Video between 2 Videos . i want to do like add black-white video at last second of **1 video** & starting second of **2 video** . overlay merge this 3 video( **black-white, last second of 1 video & starting second of 2 video**). at last concate all this videos & get output.
please provide command for this.