Adding Work to the sprint to make some of the team members busy

Hi, I work in a scrum team of 6 developers, a PO, and a Scrum Master, Usually, we start our sprint planning by adding some features to the sprint backlog, In this process the work is divided between the team members based on the expertise they have (Front end , backend..etc), and normally every 2 members work together to finish a feature, and the next high priority feature get assigned to the next 2 members and so on, but usually we end up by some of the team members have filled their capacity for 2 weeks work, while others still have the capacity to work on other feature but often there ar