How do I split a vertical video in half and make it horizontal?

A screen shot of your clip would help. In any event, you might try the procedure below to see if it accomplishes what you want.

Import the video clip into an iMovie project. Click on the project's name under the Project Media heading at the top of the sidebar. That will reveal your imported clip in the media browser. Do an Edit/Duplicate Clip. That will leave you with two identical clips in the media browser. Select Clip no. 1 in the media browser (not the timeline). It will appear on the preview screen. Click on the crop tool (the one that looks like intersecting angles) in the toolbar at the upper right of your screen. A little square with the word "Crop" in it will appear just above and to the left of the preview screen. Click on the Crop box. In the preview screen an adjustment rectangle will appear with adjustment handles at the corners and sides of the clip. Drag the handles to outline the top half of the image. Click on the blue button (with the checkmark in it) to the right of the screen to crop and preserve the portion of the clip that you outlined. Follow the same procedure with Clip no. 2, only this time crop the bottom half of the image. Now you have two clips -- Clip no. 1 is the top half and Clip no. 2 is the bottom half. Put them in the timeline. Move Clip no. 1 to the timeline above Clip no. 2, and select Clip no. 1 by clicking on it. Then click on the overlay tool (the one that looks like overlapping squares) in the tool bar in the upper right of your screen. Choose Split Clip from the overlay options box. Split the clips side by side.

-- Rich

With split screen, unfortunately, you cannot adjust the separate images.

You can try going back to your clip in the media brower of the right hand pic and do another crop of the original clip. This time, crop off more of the part that is above the speaker's head. Then overlay the re-cropped clip as a side by side pic. You might need to experiment a few times until you get it like you want.

Another way might be to put your clips into a newly created project and split them out as above. Mute the sound on the right hand picture. Share out the project and import the shared video back in. You will now have one clip that is a split screen in the timeline. Then take a copy of the cropped clip of the right hand speaker and overlay it as a picture in picture over the face of the speaker on the right hand side. You can adjust the size and position of the picture in picture. Then you can share out your work and insert it into your original project, or leave it in the present project without sharing out. . \

-- Rich

I now recall that when I did this once, I split-screened one video over a black background, so that the video was on the left of the split, and a black screen on the right. I shared it out and imported it back in. Then I overlayed the second clip onto the black side with picture in picture. With picture in picture you can move the clip around and adjust its size.

-- Rich

Glad it worked out.

In iMovie 10 the time above the timeline only measures in hours, minutes, and seconds. It doesn't measure in fractions of a second.

You can use the left and right arrows on the lower right of your keyboard to advance forwared or back one frame at a time. That might help.

-- Rich

Happy to help. If you have time, post back with your results.

-- Rich