Why is there a gap between two images that normally fit together?

[theembracedone]

I have two images that look like this in Photoshop:

but then ingame, you can see that there is a gap between them:

What can I do to fix this? Thank you

EDIT:

In the meanwhile I’ll leave this gif here to illustrate what I’m trying to achieve as an end result:

I believe I will need a shader for this