maybe between Aubameyang and Lewandowski is just for comparison in the team. If only Aubameyang there is nothing competition for the position, that will not good for building the team and the future. Lewandowski is enough experience as an attacker and it's very good for them, especially for that position, and not only as a great goal scorer, he will be good for sharing knowledge with a young players.
It is true that it is more motivating for Aubameyang to compete, but on the other hand they should not forget their weaknesses and only focus on Lewandowski. If that happens then it's useless their attack looks scary, but behind them it's easy to lose goals. Then it will be their next mistake which will add to their burden.