Mirror Image
Before drawing, trace a vertical line at the center of your page, in order to make the drawing easier. One of the partners guides the work by drawing on their side of the page, while the other draws the same strokes in symmetry on their side.
(!) To draw in mirror, you can find the strokes your partner has used by pressing the Shift key on your keyboard 
and then clicking on the desired stoke. Your stroke will thus take on the desired stroke's parameters.