You misread my post.You never need more than one layer, but sometimes more than one course. Some people go full overkill and do the whole roof with it, but double course at the most is generally all that's needed. Ice and water shield has been around for a pretty long time now and is a big help in stopping ice dam caused water incursion.
Two rows, vs to courses... its the same. I agree you don't need two layers, it's pretty heavy stuff.