Blend, merge or what?

I am shooting a documentary where I want the scene or background to remain static or the same, but where the "historic" characters fade out while the narrator/modern character fades into the scene. However, I am not sure what to call this transition so cannot find how to do it. I used Final Cut Pro and think I might be able to do it in Blend mode using two shots where the background is exactly the same. Can anyone help? Am I on the right track. Can anyone give me a nudge in the right direction?

Put them on two tracks in exactly the same position and use opacity keyframes.

Thanks Uli;
So, to be clear, is that in Blend mode in FCPX? Should I shoot this as one take, or two, or both. As long as I shoot it correctly I am good for now as I can learn the editing later. Do you know what this is called? One of the issues I have with finding information is the simple fact of finding out what the correct term is. Thanks again.

Blending in and blending out would be terms that fit. Whenever you mix two images, in your case over a period of time, it's blending.
It would be easier to advise if you could post a short example of what you want to achieve.

Thx Uli - missed your second post as I seem to be missing notifications.

So: I want to have a scene, in this case at a campfire, with 1860s characters. I want the modern narrator to walk into frame as the 1860s characters fade out, or disappear leaving only the narrator. There would be no camera movement.

Similarly I want to have my historic characters fade into a modern setting. The background will be static but the characters walking into the frame.
Make sense?

I cannot copy the example I have in mind, unfortunately.

Thanks again.