The steps
Blending is very easy and can be done in different ways (using layer masks, gradients etc.). I'm going to use only the Layer Modes and the Eraser Tool. You can download the pictures I used by clicking on the thubnail and work with them to understand better.
First of all, be sure that the two images you will blend, the other one twice wider than the first one.
Create in Photoshop a new file, same width as the second one and same height.
Open the two pictures and drag them into the new one.
Then, chose the second layer and switch the mode to Darken.
Switch to layer 1, chose a 300px eraser and begin to erase as in the picture (in the middle, where the two pictures blend together).
After doing this, switch to the second layer and with the same tool selected (300px eraser), erase the left part of the image (on the girl - to make her visible). The final result should look like this:
This is it!