Our master bathroom has a double rod. DH folds his towel in half and hangs it on the rod closest to the wall. I fold my towels in thirds and hang them on the rod furtherest from the wall. In any given area, there are five thicknesses of towel hanging there. They dry fine. After washing, I only dry towels in the dryer for 30 minutes and they hang dry on the rods the rest of the time. After showers, we just hang them up. They are always dry for the next shower and we've never had any moisture issues.
However, we live in San Diego and the climate is probably drier here most of the year. It is currently our wet season, though, and the towels still dry just fine. Air flow is really the key. We keep all doors open (except while showering) for maximum air flow. Our bathrooms have fans permanently attached to the light (at the switch), so when the light is on the fan is running and air is circulating.
"Kind words can be short and easy to speak, but their echoes are truly endless." - Mother Teresa