I draw a curve and then draw a rectangle below it. The curve stroke is 6, and the rectangle stroke is 3. I cannot move the rectangle to match the curve. It always has a little bit of a gap.
How can i join them to form a single object (path) ?
Getting them to line up is easy enough since one stroke is exactly half the width of the other.
Make sure Smart Guides are switched on
Open the a Transform dialog and set the options as shown below
Select the rectangle, and in the Stroke panel set "align stroke to outside"