They probably stripe the data across the banks of platters just like multiple drives in raid 0, so when reading or writing a sufficiently large file, both banks are always operating at full speed.
Edit: other comments below explain the missing part, in a regular drive, apparently just one head can read/write (to one platter) at a time due to calibration reasons, even if your disk physically has 8 platters. So having two actuators allows two platters out of 8 to do IO concurrently.