In order to observe two frequencies simultaneously, the beam has to be
split into two beams with the help of a dichroic. This adds some
instrumental noise that we will encode as a higher value of the receiver
noise (i.e., in increasing
by for instance 10 K).
As the dichroic can be removed from the optical path when doing single
band observations, this
increases will only happen when the
sensitivity estimation is done in dual band mode.