I wash my WW's seperatly.
For my BMW 5 series, I'll use my megs WW towel first, then when that starts to leave water behind, ill take out my poorboys blue WW, and do the rest. Then take another poorboys blue WW, and clean up areas that I may have missed.
I can use anywhere from 3-4 towels on my car.
I noticed that my towels would streak when I first used them, so I would wash them 2 times before using them to get any oils out.