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Both beat higher-placed qualifiers in the semifinals on Saturday.
Fourth-placed Maheno caused a minor boilover against top qualifier Valley at Weston Park.
Maheno took no prisoners.
It led 22-7 over Valley - last season's champion - at halftime, and held on for a 29-21 win.
At Whitestone Stadium Old Boys finally beat Excelsior - which in the regular season had beaten Old Boys on three occasions - 33-21.
Excelsior had the better of the first quarter, and should have had an early lead when normally accurate kicker halfback Tarn Crow missed a relatively easy penalty in front.
It had to wait until three minutes before halftime, when it won a breakdown and impressive first five Tom Moysey carved up the Old Boys defence to score and convert.
From the kickoff Old Boys went on the attack, and left winger Suli Tuivailala outpaced the defence for first five Inoke Naufahu to convert.
From the restart Old Boys opened up the Excelsior defence again, and Naufahu added a converted try to give Old Boys a 14-7 halftime lead.
Old Boys dominated the first 30 minutes of the second half.
Flanker Toni Taufa and Naufahu added tries, with Naufahu's couple of conversions taking Old Boys out to a big 28-7 lead heading into the final 10 minutes.
Excelsior kept testing the Old Boys defences, and in the dying stages Moysey showed his class with a second try, followed by another to winger James Pillay.
Moysey's couple of conversions put Excelsior seven points in arrears before Old Boys centre Alipate Tuipolotu added an unconverted try to seal the 33-21 win.
Out at Weston Park, Valley started confidently, with fullback Howard Packman scoring, first five-eighth Brad McKenzie adding the conversion.
But Maheno swept into action with tries to centre Clement Gasca, prop Mike Mata'afa, and electric right winger Adam Johnson.
Maheno's general Robbie Smith converted two of the tries and added a penalty to give Maheno a 22-7 halftime lead.
Valley hit back in the first five minutes of the second half when tighthead prop Meli Kolinisau touched down for McKenzie to convert.
Ten minutes into the half Smith scored and converted to extend Maheno's lead to 29-14, following some outstanding defence.
Valley fullback Packman displayed his ability to score his second try, which McKenzie converted, but Maheno was able to hold on for an upset 29-21 win.
- TERRY O'NEILL