What is the first and last name of the artist who performed back stage for the song "Der Kapitan"?
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"