overlaying object problem

Hi guys, I'm having a strange issue that I don't know how to solve, I'm using tree object, 2 cubes with an unlit material are two "skies" and another one is a block that I want to use to make a merging effect of the two skies. The issue is that I get some strange lines around the "merging block", I hope the image can explain better than me: How can I solve this problem? Since I'm using an orthonographic camera I don't understand why I get those lines