Actually, two step sparge is add the 6.4 qts before you drain, mix well, then drain.
After draining, measure how much you gathered on the first running and calculate how much more is needed for your pre-boil volume (you have to know your boil-off).
Add whatever water is needed for your second running and mix well then drain.
BS has predicted that you will need 28.88 qts to hit your pre-boil volume.
Keep in mind this is based on someone else's recipe and equipment profile, so your results may be different.