Hello friends,
I have attached a file below. It will be nice if anyone would help me to understand how was the effect done.
I know that we used mask and then did alpha; but yet this one looks bit complex.
Thanks for the help in advance.
Hey Anirban,
This is really a very easy effect to create. How to do it depends on if you want to use still images or moving images. Either way, it is still pretty simple. You stack the images one on top of the other, use size and position to place them where you want them to be, then use "mix" in the chroma key section, or just drag the video level up and down to create the intensity of each clip or photo.
Tell me if you want to use still or moving images and I will do a quick YouTube tutorial on how to do it.
George
ps. I don't think you need to use either mask or alpha unless you are trying to do something more complex than what you have on the images you sent.
Dear George,
I would like to have the effect both on still as well as moving files. I will be thankful if you are kind eough to show me the same in Youtube.
However I tried your guidelines as asked by you, but that didnt work. I can see the lines of each of the pictures and they dont overlap smoothly. I have attached the picture for my output.
Hi Anirban
For what you want to do you need masks which have a graduated fade as required between the black/white such as :
Split screen style mask
or
top right quarter fade in mask
You can create them in almost any image editing program
Note both these images are originally 1920 * 1080 pixels
John
Hi Anirban,
Here is an example of how this type of overlay can be created without using masks. The problem with it is that none of the images show a very pronounced video level. 
If you need high video levels and a graduated transition between images, masks are the way to go.
George