ONR is an awesome product. When I do a rinsless wash with it, I do use a second bucket with the same ONR concentration in it. I know it is supposed to be unncessary to use a second bucket to rinse your sponge or wash mitt, but I do anyway just to be safe.
BTW, ONR is meant to be primarily used as a rinseless wash, not a waterless wash. In a rinseless wash, yes, you still use 2 or 3 gallons of water and then put one or two ounces of ONR into it. For a waterless wash, you can put ONR in a spray bottle and use it at a higher concentration. However, Optimum, the maker of ONR, has a separate dedicated waterless wash product called OptiClean. Just put Opticlean in a spray bottle at the corect dilution ratio. No bucket necessary.
I have used OptiClean and it is a very good product. Most people would be very happy with it. I personally don't care for it quite as much as either Meguiars D115 Express Wash and Wax or McKees 37 Waterless Wash on the Go. OptiClean is white in color, so when I try to dilute it into my spray bottle, which is white in color with a clear fill line, it is harder to see the exact level of the liquid in the bottle. Meguiars is light blue in color and Mckees is orange, so much easier to see and dilute with precision. I personally like the scent of Meguiars and Mckees 37 better than the scent of OptiClean. OptiClean has a nice, pleasant scent, but just a personal prererence. And finally, I think the Opti Clean is more expensive in terms of cost per use.