Once again it was all about patience as youthful, energetic Watford were happy to string two lines of four in front of goalkeeper Richard Lee and wait for us to break them down.
That took until the 50th minute when Leonhardsen struck and Kamanan's goal 10 minutes later just about wrapped it up with Watford offering too little, too late on the attacking front.
It took 20 minutes for a slightly changed team - no Sergei Rebrov or Tim Sherwood - to settle down and then the chances flowed.
John Piercy had the first effort, shooting wide from 20 yards after a one-two with Simon Davies and we should really have taken the lead two minutes later when Lee Barnard's trickery and cross from the right flank left Kamanan with only Lee to beat from eight yards - he sliced wide.
Davies made something out of nothing on the half-hour, controlling a long ball, beating two men before squaring to Leonhardsen but the Norwegian, uncharacteristically, missed the target from 18 yards.
Ciaran Toner was next in line, forcing a low save from Lee and the visiting keeper produced a save out of the top drawer to deny Kamanan a minute later.
Ben Thatcher unleashed a 20-yard drive from a free-kick just over a minute before the break and Watford finally had their first effort on goal, a looping header from Fabian Forde onto the roof of the net, in injury time.
The breakthrough came five minutes into the second half. A fine move down the right flank involved Davies and Piercy ended with Piercy's cross being sliced in the air, Barnard pounced on the loose ball only for Lee to save but Leonhardsen was there to slot the ball home.
The lively Barnard was heavily involved in the second goal on the hour. He held off three defenders before seeing his shot saved again by Lee but this time Kamanan tapped home the rebound.
With the back three of Gardner, Bunjevcevic and Thelwell in control that looked enough but Watford became more adventurous and after Sam Swonnell struck a post, substitute Anthony McNamee turned on a sixpence before firing home for 2-1.
That was with just two minutes left and led to a couple of late scares but the points went to the right home.
Spurs Reserves: Kelly, Piercy, Bunjevcevic, Gardner, Thelwell, Thatcher, Leonhardsen, Toner, Davies, Barnard (Consorti, 70), Kamanan.
Watford Reserves: Lee, Neill, Panayi (McNamee, 88), Swonnell, Langston, Ifil, Saunders (Blizzard, 70), Williams, Forde, Godfrey, Matthews.
In almost a carbon copy of Saturday's Premiership win against Middlesbrough, Oyvind Leonhardsen and Yannick Kamanan maintained our reserve side's unbeaten record at Stevenage Borough on Monday night.