Defending champion Niall Mackenzie struggled to eighth and fourth in the double-header while Yamaha team-mate Chris Walker earned second and fifth places.
Reynolds on his Ducati hit a deep puddle while leading race one easily and Emmett took full advantage, slicing past Walker on the last lap after a spectacular duel on the slippery track. Kawasaki's Terry Rymer also timed his charge to perfection, grabbing third from Michael Rutter's Honda in the final mile.
Reynold's machine was repaired for race two, which he won by less than half a second. Tyre choice was critical on a drying surface. Emmett and Mackenzie were on intermediate instead of full wet weather tyres, and although Mackenzie fought back to beat Walker into fourth, Emmett could only finish sixth.
Honda's John Kocinski and Ducati's Pier-Francesco Chili won a race each in the fifth round of the World Superbike Championship at Monza yesterday.
Results, Digest, page 23