I would assume that the doctor is trying to alleviate your afternoon sleepiness by “heading it off at the pass,” so to speak. It may be that he wants you to take another 10 mgs at noon because by the time 2-3 comes around, your meds have worn off. This way, you don’t have to suffer the lag associated with your meds wearing off all the way before taking more and having to wait even longer for it to kick in. Make sense? Also, if you take the second dose at 2 or 3, it may be late enough to adversely affect your sleep.