Real Salt Lake opened the scoring in the 38th minute, when Luke Mullholland got behind the defense inside the box and headed in a long pass from Morales.
The 19-year-old Allen's header from close range came off a pass from Javier Morales from the right wing.
In Sunday's other game, Adailton scored the lone goal to lead Chicago over Philadelphia 1-0 for its first victory of the season.
Adailton slipped the defense in the 37th minute then deftly cut in front of goalkeeper Rais M'bolhi and nodded a header into the open goal off a pass from Harrison Shipp.
Philadelphia played the final 20 minutes down a man after Fred was shown a red card for throwing a punch at Shipp after a tackle at midfield.
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