Merge softly 2 cropped videos

Hi all,
I’m using shotcut to create a mlusic video where I play 2 guitar parts. I’m sitting in both sides of the camera for each take. Then I crop one half of each video and put them in a single output.
The problem is that the light is changing all along the takes and it’s not easy to adjust.

What I’d like to do is to have soft edges for both cropped videos so that they overlay each other slightly but you can’t see the edges of each video.

Another solution would be to adujst dynamically the light of the videos to match each other along the duration of the movie.

Here’s the result for the moment :
I tried to adjust the lighting but I don’t know (yet) how to do that with keyframes to have a soft transition from one light setting to another (but I’m sure I can easilly find tutorials about keyframes adjustments).

I think it would be easier to have a way to crop the video with soft edges like you can do in photoshop for example where the edge of the image is becoming less and less visible.

If I take screenshots of the 2 source videos, what I’d like to do is something like that for each video (this is done very quickly just to illustrate my idea, the soft edge is too large and badly adjusted between the 2 images) :

I found my solution with masks.
Based on this video :

I won’t do that one again but I’ll know for the next ones. I’ll also work with artificial light to ensure a better consistency between the two videos.