Have you ever tried to paint on a wet surface with watery color. You need to keep a couple of steps ahead with your plan and get it all on before the painting dries. One can add a couple of drops of glycerin to the water to slow the drying. It is difficult to judge the intensity of the color and, in general, it dries at a much lighter value than it appears at first. This is my attempt at a 15-20min wet on wet watercolor. The second one was done with more intense color because of the experience with the first. 
This one took even less time, and I was able to save the whites. It is uneven with the intensities but the water reflections are somewhat forgiving. An artist I know that is successful with this technique uses only Lanaquerelle watercolor paper. This is more blotter-like and gives soft edges to everything.
I think that I will need a lot of trials to get one of these wet-on-wet paintings to my satisfaction. In the meantime, I will stay with washes and dry into wet.
