Add a listed handle tie per code; bolts, nails, etc will not cut it per current code. The purpose of the tie is so both breakers will open by a manual throw of the handle. The handle tie will not cause both breakers to trip on an overload condition on only one circuit breaker. That is OK because the purpose of the breaker is to interrupt current flow under an overload condition, not completely isolate voltage from both legs. Anyone working on the device being fed by the circuit breaker needs to shut off the circuit before beginning work, hence the purpose of the handle tie.