[Path between merged shapes?]

I try to create a triangle shape. To get a symmetrical shape, I started by creating the left half of the triangle. Then I duplicated it, reflected it vertically and aligned the two shapes. Then I used the "add" function to merge the two halfs into one. Now there is kind of a path between the two objects. If I try to round the corners both objects behave like individual shapes. Is there a way to merge them into one single object?