Really how do you figure although I never seen a Xterra. Unless it has a hose coming out and a hose going in it does not cycle threw. On a closed system there is a hose that is in the bottom of the res tank that by vacuum it is sucked out or blown back into the res as the radiator level expands and contracts.
Now if you could fill the rad directly ,which you cannot on these trucks that I'm aware of ,then it will cycle or flow around and eventually mix. What is in the system now has probably "mixed" but I would NOT recommend doing it this way but mix it 50/50 before pouring in. What your creating is hot spots or did create IMO.
On edit: Plus you have no accurate way to test your protection to see if you have added enough antifreeze. Also what is the majority of the res tank,water or antifreeze? What was poured in second water or antifreeze in other words?