Patrick; Any biochemistry text will draw chemical reactions with circular arrows for each reactant. Things keep cycling each other. With an ET developer, it can either recycle both ingredients or have one 'pump' the other in activity. Both types can be ET, but the former, with total cycling is distinctive enough to be clearly an ET developer. The latter requires many tests to show if it is an ET developer. It may or may not be. It may just be ordinary superadditivity.
The clue in this in the latter case is if either agent is virtually inactive alone even at high concentrations. Placing a large amount of the most inactive into the developer, and a tiny amount of the other (which is recycled) will give a very active, long lived developer with high capacity. The one at high concentration and with low activity is being used up to regenerate the small amount of the other agent.