Hello mac-forums people!

I'm a musician and I'm looking to make some YouTube videos of myself playing, and I want to have a horizontal split screen effect on some of them so I can show myself playing more than one part/instrument at the same time.. like this:

It seems so simple, but I can't figure out how to do it. The closest I got was using the Picture In Picture tool in iMovie, but when I cropped the inserted video down, it kept the same aspect ratio, which isn't what I need.

Basically I need to crop one of them to make it thinner, but keeping the same width, so you can see the guitar, then use the Picture in Picture tool and put that thinner video at the top/bottom of my other video. Does any of that make sense? I hope so.

Any help would be greatly appreciated!