Starting by preparing Caffenol black and the Orange developer listed below,

I repared that both have the same pH value and if I mix them then the result would have the same pH value but if I mix them fifty-fifty the final solution will have half the amounts of chemicals per liter and the recipe would be then:

1000 ml water
10 g soluble cheap coffee
23g Sodium Carbonate*
5 g of Vitamine C

What changes then? The developing time, because coffe and Vit. C are superadditive. In fact the developing time drops to 12 or 15 minutes. The first roll I have developed in just 10 minutes but the film is slightly underdeveloped. Next time I will try 15 minutes. To the 1 liter developer I added a pintch of Potassium Metabissulfite to see if it helps to preserve the solution for reuse. I will test this tomorrow.

* I was lucky making this mistake. In fact, it should be 28 g, but using just 20 g, like I did, the desired pH of 10,4 is achieved too, so it works and gives finer grain but takes a little longer, 15 minutes.