Actually, all the documentation I can find on the Twiggy drive says they did not have "dual opposed heads", IE, four heads in total, but simply had a head opposed by a pressure pad on opposite sides of the spindle. (Apple's justification for that was they were concerned that opposed-head drives would wear the disk media out faster than the pressure pad would. Apparently they started work on Twiggy in 1978, before industry-standard double-sided drives were at all common.) There isn't going to to be any speed advantage in that geometry compared to two opposed heads, and in fact it's going to be slower, because Twiggy used a multi-speed motor.

(The two heads were moved by the same actuator, so when one head is near the edge the other is near the middle. Reading one head and then the other would require speeding up or slowing down the spindle by a large percentage, thus it would be quicker to step and read the next track with the same head. Once you've read one side you'll have to speed up or slow down the drive in one big jump and switch to the other head. This is going to be a lot slower than an opposed-head drive, which can simply switch heads between rotations and read twice as much data between steps.)

It's pretty astounding they didn't wise up after industry-standard drives were introduced and drop the concept. They spent *five years* banging their head uselessly on that concrete wall.