The offensive outburst was unusual for United (4-14-3), who won for the first time in three games under interim coach Ben Olsen. They had not won in eight matches, scoring just three goals.
Bill Hamid made two saves in his first career shutout and United's first shutout win this season. The Union (4-11-5) extended their winless stretch to a season-worst six games and were shut out for the first time in nine matches.
Andy Najar set up Allsopp's third and fourth goals of the season in the 22nd and 63rd minutes.
On the first, Danny Califf intercepted Najar's cross in the middle of the box but then lost his footing. Allsopp stepped in to fire from 10 yards and beat Chris Seitz on the left.
Najar's cross reached Allsopp on the second goal, and he again buried the shot on the left.
(Copyright 2010 Philadelphia Union. Reprinted by permission)