I don't think it really matters, but by using two grounds you can find a loose ground a lot easier, it will either be in P1 ground circuit or P2 ground circuit. Whereas if you use one ground then you have a lot more areas to check for loose connection.
I have never had a problem with loose grounds after the cab was "commissioned", but I always try to use two grounds, just to be more tidy looking if nothing else.
I have heard of folks using both grounds and connecting them together at the end of the loop, just to provide an alternate ground if one fails, but I would want to know right away if something in my wiring fails, whereas this method could cover up a problem until another problem surfaces.