I want unite two shapes and create a perfect smooth curve between then.
I know how to do that using Pen Tool and/or plugins (Xtream Path).
But I would like to know how to achieve this result in other ways, using only Illustrator. Maybe someone here has a simple way.
PS.: If my question is not clear I can try to explain it better.
Draw a circle to create the curve and place it appropriatly. Select all three shapes (the two rectangles and the circle) and use the Shape Builder Tool to remove and combine parts.