It is possible that you are confused with effort vs. duration but I’ll assume you understand and what you want is to split the work between the assigned resources. Set the task to task type fixed duration and effort driven selected. Assign the first resource, then assign the other resource(s). This will maintain the duration and split the effort between all assigned resources. So, a one day task will assign 1/3rd of the total hours to each resource. This will work but still depends on the details of what you are looking for. Hope that helps.