Copy and Pasting a clip

You are in the Photoshop Elements forum.  I will move your post to the Premiere Elements forum where you will get more expert help.  But to get the basics, I suggest you look at the Trim and Split Clips Guided Edit which will walk you through the process:

Essentially, you can select videos A and B in the Organizer and then choose to edit them in Premiere Elements.  The two videos will be placed on the Premiere Elements timeline.  You can move them around by click-and-dragging them into your desired order.   For example, if you want to add a portion of Video A to the beginning or end of Video B, you can delete the unwanted parts of Video A and drag the desired portion to the beginning or end of Video B.  Once your edits have been completed, you then need to render the project containing both videos to a new video file containing both.

I hope this makes sense to you.  If not, the experts in the Premiere Elements forum will help you out.