The Defiant had a crew of 50, about one-third of Voyager's. It required dual authorization.
The Defiant needs a two-person aurhorization, and it has 1/3 of the crew and it's 1/3 the size. (5 decks vs. the 15 decks of Voyager.)
So that explanation doesn't work.
In which episode? If it was after Voyager launched, maybe they realised two-person authorisation was important.