After merging 2 video some frame is missing ,

Hello, First I would like to say thanks for creating a nice code for merging, I got one issue with that, Like when I am merging 2 videos some frames is got missing.

1>  video duration is 5 seconds
2> video duration is 5 Second 

After merging it duration should be 10 but it is showing only 8 seconds.  
Could you please suggest to me what is I am doing missing?