  A variety of possibilities exist for dark matter aside from WIMPS, such as
hidden sector dark matter. We discuss synchronous thermal evolution of visible
and hidden sectors and show that the density of thermal relics can change
$O(100\%)$ and $\Delta N_{\rm eff}$ by a factor of up to $10^5$ depending of
whether the hidden sector was hot or cold at the reheat temperature. It is also
shown that the approximation of using separate entropy conservation for the
visible and hidden sectors is invalid even for a very feeble coupling between
the two.
