For two independent sets of heads to be able to work with the same sets of tracks, the positioning needs to be spot on. When you have one set of heads, it doesn't matter *precisely* where the heads go when they're looking for track 13, so long as they consistently move to the same physical position each time track 13 is asked of. But with two sets of heads, they also have to agree on what that position should be.
I remember that, years ago, this was the reason given for the much lower densities on removable media than were available with hard drives - because they were, effectively, subject to being interacted with by many sets of heads over their lifetime and so the tracks needed to be wide to allow for misalignment.
Perhaps this is less of an issue these days - not sure.
Biting the hand that feeds IT
