A few years back Conner developed a drive that had two actuators and 2 sets of heads (what the original poster is asking about) called the Chinook. The company was subsequently bought out by Seagate and I believe they were the ones that actually released the drive. Problem was, it was so complex and the performance gains weren't that great, plus the added cost was unwanted, so the technology was cancelled.
"but do they all read/write in unison across multiple platters together?"
No, only one head can read/write at a time.
"CD devices have access times around 70+ms and access time would need improvement for this type of head mount use in HD's."
CD's have horrible access time due to the inconsistency of the media. Different discs use different reflective layers with lower or higher reflective properties, scratches, dust or fingerprints on the disc, also the discs wobble differently often greatly at high RPM's, which all add up to a much tougher surface to focus on than a hard disc.
"Using head mounts like the ones used in CDs, couldn't one easily have independant heads with the necessary degrees of separation between head supporting arms [ON THE SAME PLATTER] and even apply this methodology to different platters? This could effectively [ALSO] allow the same drive to support multiple, simultaneous reads and writes from independent heads on different faces of each platter if the bus could handle the different I/O streams in parallel. The harddrive bus connections would have to be better than SCSI variants and more like multiple, parallel SSA, or one SSA channel for each head."
Optical discs and HD's use completely different methods for reading the media. Optical drives use a laser to reflect of the discs detecting pits which alter the reflection. Hard discs are magnetic media read by heads that detect the magnitism of the cluster on the disc. The two technologies have nothing in common that can be applied to the other.