First half goals from strike pair Marc Newsham and Nathan Jarman saw Gainsborough progress to the FA Cup third qualifying round at the expense of bitter rivals Boston United.
Trinity fans waited 19 years for a home win over the Pilgrims, and now two have come along at once.
Steve Housham’s men followed their 1-0 league victory over Boston in August, with Saturday’s 2-0 cup triumph.
The Pilgrims struck the woodwork twice in the second half but generally lacked a cutting edge at the Martin and Co Arena.
Boston entered the tie on the back of one win from six but started brightly only for Trinity to gain the upper hand in this Lincolnshire derby.
Jarman and Newsham caused plenty of problems for the visitors defence in the first half, Newsham’s lay-off seeing Jarman just wide from the edge of the box.
Jay Rollins was in at the other end but as he attempted to go around Jan Budtz the Trinity goalkeeper made an important save.
Rollins then limped off, Gainsborough making the breakthrough midway through the first half when Jamie Yates’ long throw saw former Boston striker Newsham turn to fire past Alexander.
A close range Carl Piergianni header brought out the best in Budtz but it was Trinity who were in the ascendency for the rest of the half.
Alexander, replaced by Henry Ravas for the second half, fumbled a Jarman curler, while penalty claims for a foul on Jarman were turned down.
The second arrived six minutes from the break when Jarman scored with a free kick from just outside the box, Budtz then saving well from Scott Garner.
Boston came out strongly in the second half, hitting the bar twice in two minutes as the home defence failed to deal with two Nicky Walker corners.
Piergianni was unlucky with the first and Dale Southwell the second, Gainsborough then controversially denied a third against the run of play.
Garner gave the ball away cheaply and Jarman homed in on goal only to go down under a challenge from Boston’s replacement keeper Ravas, referee Paul Graham unmoved.
The impressive Budtz tipped Walker’s 20-yard drive past the post while Trinity sub Nick Bignall raced onto to Dominic Roma’s ball over the top only to shoot into the side-netting late on.
Gainsborough Trinity: Budtz; Roma, Quinn, Picton, Lacey; Davis (Brogan 90), Russell, D’Laryea, Yates; Jarman (Bignall 68), Newsham (Forrest 77)
Subs: (not used) Stamp, Binns, Wilson, Hedge GK.
Boston United: Alexander (Ravas 46); Roberts, Mills, Piergianni, Marrs; Rollins (Jones 18), Garner, Dixon, Greene (Richards 72); Southwell, Walker
Subs: (not used) Dowie, Johnson, Limb,
Referee: Paul Graham