Fresh from their 8-0 win over Darlington on Monday, the home side defeated the Pilgrims 9-2 at Wensleydale Park.
Flynn Slater and debutant Callum Pratt scored United's goals.
Boston (3-4-2-1): Jake Lovelace; Taylor Redman, Owen Sheriff, Lucas Stubbs; JJ Ward, Jake Henderson, Rueben Fogg, Josh Doran; Han Stevens, Krisztian Gagyi; Flynn Slater. Subs (used): Kyle Featherstone, Will Langford, Callum Pratt.
Referee: Dave McCallum.