Carol you don't want 20 lbs of Harrisons pellets, that is too much at a time for two greys, because Harrisons is organic it has a shorter shelf life so you don't want to buy more than you can use up in a couple of months. I would buy the 5 lb bag or maybe two 5 lb bags and leave one sealed up until you need to open it.
I don't think Josey has ever eaten any other pellets than Harrison's high potency, I am sure there are others that may be just as good but it is recommended so much by avian vets that I feed her that exclusively.
I don't buy in bulk as I only have two birds and I get almost all of it online, except for Harrisons I get most of my supplies at www.drsfostersmith.com