Trap a line

Select the line to be trapped.

In the Stroke box in the Tools panel or the Color panel, assign the stroke a color of white.

In the Stroke panel, select the desired line weight.

Copy the line, and choose Edit >Paste In Front. The copy is used to create a trap.

In the Stroke box in the Tools panel or the Color panel, stroke the copy with the desired color.

In the Stroke panel, choose a line weight that is wider than the bottom line.

Choose Window > Attributes.

Select Overprint Stroke for the top line.

Copy of stroke used for spread trap

Illustrator