i only train twice a day to fit the mileage around other commitments, so, if i get time to run at lunch or in a morning as well as afternoon/evening but usually i'd prefer, say, an 8 mile steady run than two four mile runs. however, the benefit is still the same, as long as do at least one continous long run each week, which i manage on wednesday or sunday.
i use two a day if both runs are relatively easy, or sometimes a morning run to loosen up before a speed session in the evening. I think once you go beyond 50 mpw some two-a-days are inevitable, and i'm just building up at the moment to like 60-70mpw.
when i was at university i sometimes trained twice a day and had no ill effects, but no miracles happened either. it's only now after returning from a few years off from competitive running [averaging about 20 easy mpw] that i have started to make big improvements, but that also might be because i started a fairly intensive weights regime over the summer.