Go to Edit Mesh>Split Polygon Tool. Make sure you press "reset tool" in case you have changed settings. Then click on vertex 1.....then click on vertex 2...tap 'enter' key. If it is success then a new line will be drawn between the points...as per my picture.
Hope it helps
cheers bullet
bullet1968
"A Darkness at Sethanon", a book I aspire to model some of the charcters and scenes