In Mario Kart Wii, karts can get a second level of mini-turbo by holding a drift longer before letting go. Is this any better than getting two regular mini-turbos on the same turn? If so, how? Does it provide more than double the boost? Does it take less than twice as long to get?
If speed is the only factor, I would say they are about the same. I spent some time playing tonight, running time trials on DS Yoshi Falls in a standard S kart. I ran the track about a dozen times, using the same route through the course. Half the time I would drift through the first turn long enough to do a double mini-turbo. The other half, I would use two regular drifts through the first run.
Based on my internal timer (i.e., I didn't use a stopwatch, just counted things off in my head), you have to drift 2 to 3 seconds to charge a regular turbo, for which you get a 0.5 to 1 second boost. The double mini-turbo requires a 4 to 5 second drift, for which you get about a 1.5 to 2 second burst. So, two regular boosts are roughly equivalent to a double.
I give a slight advantage to the double mini-turbo, however. Because you have to drift longer to charge for a double boost, you spend more time with your kart under control, holding the tightest line. Doing two regular boosts, you tend to lose your line between the two, which costs about a half second in lap time.
Of course, most of the turns in the game aren't big enough to allow drifting for 5 seconds, so a mini-turbo is what you'll use 90% of the time. But for big turns, hold your drift as long as possible. Driving the tightest, shortest line is what makes the overall difference.