Independent radio circuits in the phone tend to actively try to connect the sim cards to their corresponding cell towers. As a result, 2 power circuits are being used simultaneously which results in battery draining faster. Even when one of the sims is on standby mode, it still tries to communicate with nearby towers in order to allow emergency calls in case of absence of signal. That is why dual sim phones tend to get discharged much more quickly than the ones that only have a single SIM card.