Who is performing in the back stage position for the song "Der Kapitan"? Show the first name and last name.
select t2.firstname, t2.lastname from performance as t1 join band as t2 on t1.bandmate = t2.id join songs as t3 on t3.songid = t1.songid where t3.title = "Der Kapitan" and t1.stageposition = "back"