The 4/8 double is the most difficult double to achieve (in events that are next to each other in disatnce). Thought I better say that before some clown says the 100m marathon double would be harder!!
Anyway, the difference in speed between world class 400m and world class 800m running is the largest in % terms. This means that it is unlikely that 1 person would be able to be the best in the world at both events.
History and the current situation backs me up on this and In my mind Juantorno is the only man in last 30/40 years to do it at the world level, where as all other doubles have been achieved on more than 1 occasion.