Mixed host emissive layers have been widely employed to achieve high efficiency phosphorescent organic light-emitting devices (OLEDs), and are particularly promising for reducing efficiency roll-off and improving lifetime. These co-host devices are known to yield broad recombination zones, and hence reduce exciton density. However, the relationship between co-host properties and degradation is not well understood, and there are no established co-host design principles to optimize lifetime. We apply in situ measurements of photoluminescence (PL) efficiency during device operation to understand how properties of mixed hosts influence PL and electroluminescence (EL) degradation. The wide recombination zone in these devices is found to directly mitigate PL degradation while not significantly decreasing other loss mechanisms, such as exciton formation efficiency. Notably, the kinetics of degradation in PL and exciton formation are found to differ, suggesting distinct degradation mechanisms. An important implication of this finding is that a single mechanism cannot be assumed when attempting to model OLED degradation, and that losses to exciton formation efficiency may originate outside the emissive layer. These results yield deeper insight into degradation mechanisms in co-host OLEDs, and may suggest different roles for excitons and polarons in overall device degradation.