England Lions 190 (Alsop 44, T Curran 40) beat UAE 90 (Mustafa 45, T Curran 5-16, Leach 3-7) by 100 runs
Curran took 5 for 16 to follow a handy 40 with the bat, and Leach ended with 3 for 7 from 6.2 overs as the UAE lost their last nine wickets for 27 inside 14 overs. They face a final match on Monday with the series settled.
The home team seemed to be heading for a comfortable win to square the series at 63 for 1 after the Lions had struggled to 190 all out with 15 balls of their innings remaining.
But after Lancashire's Liam Livingstone had broken the UAE's second-wicket stand with some occasional offspin, as Ghulam Shabber pulled him to Keaton Jennings at midwicket, Leach and his Somerset team-mate Craig Overton piled on the pressure.
Leach had Shaiman Anwar caught at the third attempt by Joe Clarke, and Overton then produced an athletic piece of fielding to run out Mohammed Qasim from short fine leg.
Then Curran, who had taken the first wicket in a lively four-over opening spell, returned for the 23rd over, and proved irresistible in a burst of 4 for 9 in four overs.
"You have some days in your career that stand out and I'll remember this: taking five wickets for the England Lions," said the 21-year-old, who is relishing his return to Dubai with the Lions after starring in their T20 victory over Pakistan A last December.
"They had a bit of a partnership but we stayed calm, and when I came back for a second spell I had a chat with the skipper and we said let's go for it.
That skipper, Keaton Jennings, will now fly to India to join the senior England squad ahead of the fourth Test in Mumbai. Leach took the key wicket of Rohan Mustafa, the UAE opener who was seventh out for 45, caught off a top-edged sweep. The Lions made five changes from the team who won the first match of the series on Thursday, with Leach, James Fuller, Nick Gubbins and Tom Helm making their debuts.
But Jennings lost his second toss as captain, and after losing Gubbins to a top-edged pull in the fourth over the captain fell for 21 in the 11th, edging a catch to the UAE wicketkeeper, the Lions were 48 for 2.
Tom Alsop, the Hampshire youngster making his second Lions appearance, responded positively by hitting seven boundaries in an innings of 44 from 46 balls.
But wickets continued to fall steadily, with Clarke, Livingstone and Ben Foakes all out cheaply to leave the Lions tottering at 90 for 6.
Then Fuller joined Curran in a sensible seventh-wicket stand of 53 in 13 overs that was to prove crucial later on. After Fuller went for 35 from 49 balls, Overton joined Curran to hit 23 from 35 balls including the only two sixes of the innings, before the innings quickly subsided.
"I want to make the most of the time out here to work really hard on my batting," added Curran. "I've been given the responsibility of batting at No. 7 and I really enjoyed it. I was just disappointed to get out when I was ready to pull the trigger."
Kevin Shine, the ECB's fast-bowling coach who is part of the Lions staff in Dubai, praised Curran for "bowling with pace, skill and aggression to turn the game".
Shine also highlighted "an assured debut" for Helm, the Middlesex quick, who was fast-tracked into the Lions team from the Pace Programme having impressed in the nets in Dubai.