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A variety of possibilities exist for dark matter aside from WIMPS, such as hidden sector dark matter. We discuss the synchronous thermal evolution of visible and hidden sectors and show that the density of thermal relics can change O(100%) and ∆Neff by a factor of up to 105 depending on 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.