The Republic of Ireland striker, 24, swept home Jimmy Kebe's low cross to open the scoring in the 57th minute and then won a penalty which Ian Harte converted to send Scunthorpe closer to relegation and seal a seventh-straight win for the Royals.
Alan Knill's men claimed a shock 4-1 home win over leaders QPR on Saturday and had the better of the first half before their spirited resistance was broken after the restart.
There was an air of resignation among the Glanford Park faithful at the final whistle but the jubilant Royals supporters headed back to Berkshire dreaming of a top-flight return.
Brian McDermott has fashioned an impressive, vibrant team and they have built up a head of steam at just the right time.
They showed no little character to ride out the first-half storm where Scunthorpe posed the greater goal threat and created a series of decent openings, with striker Joe Garner at the hub of their best moves.
The striker lashed a cross from Mark Duffy wide and then saw a fierce 12-yard strike being deflected out for a corner.
The visitors offered little as an attacking force in the opening half and Scunthorpe went close again in the 20th minute when Chris Dagnall dragged a low right-foot shot wide of the far post.
Reading almost struck the opening blow out of nowhere, however, when Jobi McAnuff found the space to hit a low 25-yard shot which forced Iron goalkeeper Joe Murphy into a low save to his right.
Scunthorpe remained largely on top as the interval approached and Royals goalkeeper Alex McCarthy made a smart parried save to repel Duffy's 12-yard shot in the 38th minute.
Two minutes after the restart, Dagnall showed delightful skill to break clear down the left flank and cross to the far post where Ramon Nunez could only drill the ball wide from 10 yards out.
Moments later the previously anonymous Kebe hit a sweet left-foot strike which look destined for the top corner until Murphy palmed the ball away for a corner.
With 57 minutes played, Reading struck the opening blow when Kebe found space down the right flank and his teasing low cross was swept home at the near post by Long.
The Irishman was mobbed by the visiting supporters as he celebrated and it was game over in the 63rd minute as the Royals claimed their second goal when Long was brought down by Iron captain Michael Nelson.
Harte made no mistake from the penalty spot and, although Dagnall and Nunez went close from distance for Scunthorpe, Reading's victory did not look in doubt.