How to connect two objects as union precisely?

I'm now reading this tutorial which describes how to create a simple to-do list iOS icon. On the tutorial, the author combines two rectangles as union in Sketch 3. However, when I tried to duplicate the project, it seems that I cannot combine the two rectangles precisely no matter how I try to be close to the precise point possible.

Here's my project. You can see that there is small disconnection on the combination point. I just duplicated the project with the exact same size and degree.

And of course even after I combined them as union and then make them flatten, the two objects never merge properly. I also tried to use grid but nothing changed at all.

So how can I combine two rectangles precisely, much like the author did?