The "problem" here, if there is one, is that Fusion tries, whenever it can to combine points. It tries to avoid a "point stack" in most cases. You can see here that there is just one point with 4 connected lines:

I'm not sure why there are only 3 coincident constraints shown (there should be 4, I believe, one for each line connected to this point). Fusion does not recognize this as a rectangle, just 4 lines connected together with constraints between those lines. When you create the second rectangle, picking the point of one rectangle as one of the new rectangle points, Fusion just reuses that point as one of the points in the new rectangle, and connects the new lines to it, just the same as if you create one line, then restart the line command, picking one endpoint of the first line.
I understand that this is not what you are expecting, but I'm just describing the way it works. The short answer is there is no way to decouple these when they are created this way.
To make things more confusing, point merging is not done with the point-to-point coincident constraint. It does leave the point stack in place. And, then, you can decouple them, though it is trial and error to figure out which one to delete:
Jeff Strater
Engineering Director