Hi Jim,
Before I answer there is a question you have to answer. How much light do you like. I have a customer who likes a lot of light and insisted on a layout that was probably a third more light than I think he needed. I know though that he is happy with the outcome and really does like a lot of light. So no matter what anyone else thinks you need to do whats best for you.
That said in my opinion with a room 20 feet wide and vaulted ceilings, 2 rows is a bit lean. The minimum I would layout is 3 rows. With three rows the lights would have 6 feet spacing and 4 feet off the walls. My goal in lighting a room for general lighting is to try and cover the entire floor area with good illumination, two rows I think will be too little.
Hope that helps
Paul
Light is not so much something that reveals, as it is itself the revelation.
James Turrell