How to merge two pictures in portrait mode into a single landscape photo in imovie?

There is no specific feature in iMovie 10 that lets one adjust the split screen pics to eliminate the black line. There are, however, some workarounds that can accomplish it.

Take a look at this thread for one way to merge the photos with split screen:

https://discussions.apple.com/thread/250213766

Rather than the above method, I prefer to make a side by side composite photo using the Preview app on the Mac, then take a screen shot of it and insert the screen shot into iMovie. Here's the way to do that:

1. Create an iMovie project.
2. Place your two pictures on the desktop.
3. Open pic #1 with the Preview app on your Mac.
4. Do an Edit/Copy.
5. Do an Edit/Delete. This will create a blank screen.
6. Do an Edit/Paste to reinsert the picture that you deleted. Drag the corners to make the picture smaller.
7. Leave open pic #1, and open pic #2 in Preview.
8. Do an Edit/Copy of pic #2.
9. Do an Edit/Paste of pic #2 into the open Preview of pic #1.
10. Adjust the sizes of the two pics and place them side by side.
11. Take a screen shot of the composite photos (Command-Shift-4 and drag the cursor to outline the two pics, then release the cursor).
12. Import the screen shot into your iMovie project. You can use the crop feature to make any adjustment that you need.

There are other ways to do this as well. For example, in the Photos app you can create a test photo book project and find some side by side templates in which to insert your photos, and then take a screen shot of the composite.

-- Rich