After sealing an opening day win over the Pilgrims on home soil, the visitors completed the double with an eight-goal success on the Jakemans Community Stadium 3G.
United were only 1-0 down at half-time, but suffered a second half collapse.
Boston: Alfie Harney, Finley McDaid, Jack Dixon, Josh Panton (Ben Beasley 80), Freddie Loun (Matas Gecas 30), Elod Nadasdi, Aron Ealham, Toby Foreman (Lewis Horne 80), Freddie Shinn, Sam Finch, Joe Todd (Lake Taylor-Peak 75). Sub (not used): Oscar Broom (gk).
Referee: Trevor Basker.