Sunday's results leave nothing between Arsenal and United in the early going, with the two sides tied on 12 points and having scored 11 goals scored while conceding none.
Waelti opened the scoring for Arsenal in the 15th minute, snapping up a clearance from a corner and belting it into the net, and Norway international Maanum made it 2-0 seven minutes later with a powerful run and finish.
In Sunday's late game, champions Chelsea had to battle hard on a sodden pitch but eventually won 2-0 away at Brighton & Hove Albion to stay third, level with the leaders on points but behind on goal difference and having played a game more.
Also on Saturday, Jamaican striker Khadija Shaw scored either side of the break to help Manchester City to a comfortable 3-0 win over Tottenham Hotspur.
Register now for FREE unlimited access to Reuters.com RegisterReporting by Philip O'Connor, editing by Pritha Sarkar, Clare Fallon and Christian RadnedgeOur Standards: The Thomson Reuters Trust Principles.