I stained my deck two days ago. I applied it with a paint roller. When I was done I was doing research and read that you should go over everything with a brush. So I started to put a second coat on. It is the summer and was getting really hot, so I stopped because it is not to be applied in over 80 degree weather. It is now two days later and the part of the deck that got a second looks darker and frankly nicer. Can I apply a second coat to the rest of the deck, mind you it will be three days later?
A second coat may safely be applied several days later. I will always recommend two coats to any surface unless there is a specific reason to not. You have noticed the reason why. In paints, you will achieve at most one-third as much film after it dries as when applied. With stains that will be closer to one-tenth film after drying. A second application will bring out the tones of the wood much better.
Remember, thicker is not better. Smooth even application is best.