  In this study, we investigate quantum nonseparability between an observed
system and a measuring apparatus, or multiple measuring apparatuses. We show
that the physical meaning of the outcome of the measuring apparatus obtained by
weak measurement with a post-selection differs critically from that without any
post-selection. In this study, the nonseparability plays the essential role,
which is shown to be the same in a simultaneous conventional von Neumann-type
measurement of multiple observables. From this viewpoint, we suggest a new
concept, known as {\it simultaneous readability}, which is the possibility that
multiple measuring apparatuses will give the proper information of the observed
system simultaneously. Next, we show that different components of the spin of
an electron are not simultaneously measurable even if it has EPR correlation
with another electron.
