Redmine

When I assign a person to a task, I'd like to be able to say: "This person works on this task 50% of their time". So the "effort" might be 20 engineering days. But the calendar time would be 40 days. I don't see how to configure this is Redmine? Is it possible?

I'd like to look at the "load" on each engineer and see who is at 80%, who is at 110% etc.

Thanks.