Generally, the secondary lock is recommended to add a low-cost security layer and thereby slows down the intruders. So, they are sold in combination of deadbolts and door knob locks. In case, if your door won’t have knob lock backing the deadbolt then one can easily find the interior options like slide latches, chain locks, etc. which acts as best supplement measures. However, they are essential for weaker doors like basement doors, sliding doors and other entryways which won’t accommodate the total secure deadbolt.