It's always tempting to aim for efficiency. Why wouldn't you? It doesn't make sense for people to be idle when the could be busy. Maybe this idea came from Microsoft Project where Project Managers would struggle for hours (or days) to ensure each resource on their project was 100% utilized, all the time.
In Agile, efficiency is a fallacy.
Look at it this way. What we're after is working software. Completed stories get us to working software. And even though most people won't admit it, inefficient use of resources is sometimes a better way of completing stories.
Let's say 5 individuals are working on separate tasks related to a story. These individuals communicate when needed and possibly complete the story by the end of the sprint.
But what if they worked on all the tasks (i.e. the entire story) together. Some individuals would be idle most of the time. The difference is that they're present to provide information when called upon. Nobody has track to them down. Furthermore, there are times when individuals possess knowledge that nobody else realizes. So the story progresses with very little interruption and can be completed in a day.
Am I suggesting Mob Programming. ABSOLUTELY.
But you don't have to go that far. Just stop focusing on efficiency and focus on effectiveness. Effectiveness is all about story completion which leads to working software.