The different ways to create a double line for stitches.

There are a few ways to create a double line, some editable some not. Here are the ways I have discovered so far. It's up to you to choose the best method for you.

You could use the Outline Path method:
This involves enlarging the line, outlining its path, giving it no fill and a coloured stroke.
Outline method

Or you could create a custom brush:
By making a square guide shape, drawing the pattern you want repeated inside, removing the stroke colour of the guide square and adding to the Brushes palette.
Brush method

Or using the Appearance panel create a line that has two stroke paths, one above the other.
Appearance method

Or use something called a Knockout:
Knockout method

These are arranged in order of difficulty from easiest to most complicated and some are more flexible than others.