It's quite possibly because 60mg is the recommended daily maximum dose and that is the highest that he is comfortable prescribing.

However it is quite possible that the 2 + 1 would work almost as well as 2 + 2 because even though the effect isn't there at 5 hours, there is still a good amount of methylphenidate in your system (due to the second release occuring at about 4 hours after the first dose) and the extra dose will boost it up to therapeutic levels.

Worth a try anyway.