Stack the clips on top of each other (doesn't matter which is on top or bottom), and apply the Linear Wipe effect (in the Video Effects > Transitions category) on the top clip. Even though this is a "transition", it's not an auto-animating one like those in the Video Transitions bin; you need to keyframe it to get it to actually transition. This makes it perfect for your purpose. Adjust the Completion % to create the split, then adjust feather to blend them.
Note that you may need to change the angle 180 degrees, depending on which clip is on top.
Nice tip. I would have gone it with Track Matte Keying, but this is so very simple.
Linear Wipe is my favorite hidden gem. For having only three parameters, it's pretty versatile. It's my go-to for a lot of simple masking/matting/cropping effects.
Here is my suggestion: Edge Feather.
Take the couple on the right of the frame, and move them to the left of the frame. Take the couple originally on the left side, and move them to the right side. Apply the edge feather to both images, and move them together so that they blend smoothly.
I've done it this way a few times, and I also like to Start with the opacity stronger on 1 side, and then gradualy switch it so the other side's opacity is stronger.