I would not attempt to track attendees in Project. The method of reducing duration by assigning more trainers is to make the task effort driven. Split the screen (View tab, check details) to show the Task Form in the lower pane. Ensure the training task is effort driven and not fixed duration (fixed units or fixed work). With an original assignment of one trainer and 20 hours of work as you assign the second trainer, the work should be split evenly between the the trainers and duration will drop.

The issue with tracking attendees, is they do not make duration drop. For them, the task is not effort driven.

I hope this helps.