  Of all $B \to D \overline{D}$ decays, the $B^0 \to D^0 \overline{D^0}$ decay
has the smallest observed branching ratio as it takes place primarily via the
suppressed $W$-exchange diagram. The $CP$ asymmetry for this mode is yet to be
measured experimentally. By exploiting the relationship among the decay
amplitudes of $B \to D\overline{D}$ decays (using isospin and topological
amplitudes) we are able to relate the $CP$ asymmetries and branching ratios by
a simple expression. This enables us to predict the $CP$ asymmetry $C_{00}$ in
$B^0 \to D^0 \overline{D^0}$. While the predicted central values of $C_{00}$
are outside the physically allowed region, they are currently associated with
large uncertainties owing to the large errors in the measurements of the $B^0
\to D^0 \overline{D^0}$ branching ratio ($B_{00}$), the other $CP$ asymmetries
$C_{+-}$ (of $B^0 \to D^+ D^-$) and $A_{\text{CP}}$ (of $B^+ \to D^+
\overline{D^0}$). With a precise determination of $B_{00}$, $C_{+-}$ and
$A_{\text{CP}}$, one can use our analytical result to predict $C_{00}$ with a
reduced error and compare it with the experimental measurement when it becomes
available. The correlation between $B_{00}$ and $C_{00}$ is an interesting
aspect that can be probed in ongoing and future particle physics experiments
such as LHCb and Belle II.
