How to use join command without extra lines apear

Hello, as in the title, I struggle with the Join command a lot.

for example: in the first imageI drew 2 adjacent rectangles but as you can see in the red circle, their gradient doesnt behave the same. To fix that and make the gradient apply the same way for both of them, I tried to join them together. As seen in the second image, Join command worked with the gradient but added one extra line in the middle.

What should I do in these kind of situations?

Thank you all!