i would say... it depends.

for training purposes, two a days were awful for me, especially on "hard" days. mentally, it is very difficult for me to go all out in the first session knowing that i have another session later in the day. then, the second session craps out because i justify my fatigue on the earlier session. i would frequently end up just scrapping the second session. plus, i had difficulty recovering for the next day because the warmup/cooldown needed to be done twice instead of once. so basically - decreased recovery time, decreased high end training = worst of both worlds.

obviously, when i commute, i ride twice a day, but those are relatively low intensity rides and are done just to add mileage/get me somewhere. i think for recovery, base building, or just beginning to improve fitness, two a days would not be so bad, particularly if the option is to ride once for x miles or ride twice for 2x miles. if possible, it would probably be better to do 2x once instead of x in the morning and x in the afternoon.

transitive property yields:

10 miles one ride > 10 miles two rides > 5 miles one ride