There are several ways to use multiple PLC Processors to control multiple processes, and have multiple HMI's for inputs. One basic way is to have individual processors/PLCs control small processes, and link them with some network so that they all work together. I have seen this done for speeds sake, on fast assembly machines.
Another way, is with redundant controllers. Think of two or more processors running the same program attached to the same I/O. In this case, if one processor fails, then another keeps the machine running without a noticeable change. DCS uses redundancy. CLX can be configured for redundancy too.
I have distributed multiple copies of a single HMI project throughout the mill. In those cases, I limited which stations had the right to make changes, but all could monitor the system.
DCS systems were developed for processes that have lots of instrumentation loops. PLC's were developed for lots of discreet I/O. Over the years both have reached toward a middle ground. Still, both have their place in the world. I'd hardly want to control a high speed lumber sorter/stacker with a DCS. Likewise, I'd run for the hills if someone told me that I needed to control the back side of a paper mill with PLC's. (pulping process)
I know, incoming. :0