Evan Richard earned the shutout for the Capitals, who jumped out to an early 3-0 lead.
Logan Vandervelden (1-1), Austin Woodard (1-1), Jeremy McKenna (1-1) and Spencer Camus scored for Summerside. Tayler Read (2) and Liam Kerwin drew assists.
Colby Landrigan was between the pipes for the Red Devils.
The Capitals, who won the series opener 4-2 at home on Tuesday night, will have an opportunity to sweep the series in Game 3 at Credit Union Place in Summerside on Saturday at 7:40 p.m.