Why drill twice?
Put the scrap on the door, mark and drill.
Why make things more difficult?
Trent
Drill hole in scrap, center scrap, drill hole in door.
That is drilling twice.
Put scrap on door, mark and drill.
That is drilling once.
-Jack

In all this discussion about how to enlarge an already
cut knob hole, I have not seen any mention of using a special
arbor that takes two hole saws. McFeely's sells an arbor that
accepts one saw the size of the existing hole and one saw of the
larger diameter to be cut. The smaller saw guides the drill
while the second does the cutting. Barring the purchase of a
new arbor, why not just make a dowel the size of the existing
hole and having a center bore the diameter of the pilot drill
for the hole saw. Slip the dowel onto the hole saw arbor, and
use as a guide through the existing hole.