Alignment isn't an issue - there's no alignment on a modern drive. Instead, at the factory, they write a set of servo tracks all over the platters which do the aligning for you - basically the head seeks to approximately the right position and starts reading, and the servo track tells it where it actually is, so feedback gets the head to the right track.
Sigh. Alignment is an issue, because each platter has its own alignment. That means that when you're reading/writing one platter, you're not aligned for the other platters. That's why you can't have multiple heads on one armature (which has multiple arms, all fixed together) and read/write multiple platters at once.
the bigger reason why two actuators didn't work is far simpler - think multiprocess programming. Both actuators could read or write data to the platters (of which there was one set) and if you screwed up the order of the accesses, you could easily write the wrong thing
You're being ridiculous. That's true no matter how many actuators you have — if you screw up, you write the wrong thing. Even if you only have one actuator, if you write the data to the wrong sectors, you're gonna have a bad time. But both actuators have the same job: write some data to someplace. The two don't have the job to write the same data. If the drive gets a command to write data to a sector to which it already has cached data waiting to write, then hopefully it just throws away the first command anyway. This is something we would hope any drive with queuing would do whether it has 1 actuator or a dozen.
think you do a read then a write of a sector - and the sector happens to be under the actuator doing the write
HDD sectors are either 512 bytes or 4kb. In the former case they are often smaller than filesystem blocks and there is no need to read them before writing. You just run right over them. In the latter case, they are typically the same size as filesystem blocks (we use bigger blocks on larger filesystems, and we use 4k blocks on multi-TB drives) and again, there is no need to read them before wrtiting. You only have to find them, which means waiting the seek and then for some fraction of the time it takes the spindle to go around once. Then you can write. This is true no matter how many armatures are reading/writing the same disk.