Drawing a line over another line

Hallo,

Q: Is there a fast way that I can draw a line over another line that I have drawn so that the two lines are on top of each other?

Example:  I draw a curve that I close and fill with a colour. This curve has a lot of bends and twists in it.  I then want to draw another curve next to it.  The second curve shares a section (or should I say borders right next to) of the line of the first curve.  For that part I have to spend a lot of time to draw the second curve right on top of the first curve.  Is there a fast way to do this?

PLEASE HELP!