S. Ackermann \emph{et al.}, "Simultaneous Operation of Two FEL Undulator Beamlines at FLASH", in Proc. 3rd International Beam Instrumentation Conference, Monterey, CA, USA, paper MOPF22, pp. 103–105, ISBN: 978-3-95450-141-0, \url{http://jacow.org/IBIC2014/papers/mopf22.pdf}, 2014.