I have a business case where one part of my process needs to be executed by multiple different participants concurrently. I believe that the "Multiple Instances" object should be able to give me this functionality, but I cannot find anything that clearly states if it will iterate over all entries of its collection object sequentially or if it will allow them to be processed at the same time in parallel with each other. Can anyone clarify the behavior?
it will iterate sequentially but you can do the actual work asynchronously so the iteration will be almost instantaneous. See this section of docs regarding concurrency.
HTH