Christian Iddon edged out Richard Cooper by 0.36 seconds in a thrilling race to secure his first Sunflower Trophy success at Bishopscourt.
The Tyco BMW rider led for much of the race but Cooper hit the front on lap 10 before Iddon made the decisive move on the 12th and final lap to take victory.
Eglinton competitor David Allingham took third on his Buildbase Suzuki.
Iddon, 34, won Friday's Superbike race but Cooper came out on top on Saturday after Iddon ran off the track.
Nottingham rider Cooper, last year's Sunflower Trophy winner on his maiden appearance, took the chequered flag from Gerard Kinghan and Allingham.
Newly-crowned British Superstock 1000cc champion Cooper later set the fastest lap of the feature race at the County Down track on his Buildbase Suzuki in cold, dry conditions.
Also on Saturday, Richard Kerr won both Supersport outings, taking victory in the re-started race one from Ross Patterson and Alastair Seeley after the initial attempt to run the race had been red-flagged.
Eunan McGlinchey was leading on the penultimate lap when he slid off his McAdoo Racing Kawasaki.
Kerr doubled up by coming home first in race two ahead of McGlinchey, Patterson and Seeley.
Aghadowey's McGlinchey, the 2018 British Junior Supersport champion, took the chequered flag in both Supertwins races, with KMR Kawasaki-mounted pilots Jeremy McWilliams and County Tyrone schoolboy Cameron Dawson second and third on both occasions.
Course lap record holder Iddon, appearing at the meeting for the first time since 2016, saw off Cooper by just 0.883 seconds in Friday's 'big bike' outing held in cold and damp conditions.