  Entangled pseudoscalar neutral meson pairs have been used in studying CP
violation and searching CPT violation, but almost all the previous works
concern $C=-1$ entangled state. Here we consider $C=+1$ entangled state of
pseudoscalar neutral mesons, which is quite different from $C=-1$ entangled
state and provides complementary information on symmetry violating parameters.
After developing a general formalism, we consider three kinds of decay
processes, namely, semileptonic-semileptonic, hadronic-hadronic and
semileptonic-hadronic processes. For each kind of processes, we calculate the
integrated rates of joint decays with a fixed time interval, as well as
asymmetries defined for these joint rates of different channels. In turn, these
asymmetries can be used to determine the four real numbers of the two indirect
symmetry violating parameters, based on a general relation between the symmetry
violating parameters and the decay asymmetries presented here. Various
discussions are made on indirect and direct violations and the violation of
$\Delta {\cal F} =\Delta Q$ rule, with some results presented as theorems.
