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.
Greg has it right if the "videos" are in one project. If you mean "projects" when you write "videos", Premiere Elements does not allow cutting part of one project and pasting it into another. There is a 'work around' if you need it.