Need help with cut mask

I have a question : for an animation I need to make a transition between two images. To do this, I'd like to create a thick line that would cut the first image in two, revealing the image underneath under its thickness. So I added a trim path on my pen trace and now I'm stuck.

I need your help, I've also tried setting it to "Silouhette alpha" but that doesn't work either. Does anyone have an idea?

Thanks