It happened in 2019 (49ers w/ Garoppolo and Ravens w/ Jackson). Unless you count Garoppolo’s time as a backup with the Pats. Both QBs went in having never started a playoff game before. Same thing in 2006 (Bears w/ Grossman and Chargers w/ Rivers). I assume it doesn’t include time spent as a backup or else Darnold wouldn’t qualify (he was a backup for the 2023 49ers who made it to the Super Bowl so he’s seen playoff victories from the bench).