McEwen (2-0) scored a triple in the fifth end en route to his victory over Koe (1-1) in Draw 3 — after beating Brad Gushue (0-2) of St. John's, N.L., 9-3 in Draw 2 — to take a share of top spot in the standings after the first day of round-robin play.
Glenn Howard, of Penetanguishene, Ont., is also 2-0 after ending his day with a 10-4 win over Gushue in Draw 3, scoring four points in the ninth end to secure his win.
Earlier, Howard scored a single in the 11th end to edge John Morris (1-1) of Calgary 7-6. Morris downed Jim Cotter (0-2) of Vernon, B.C., 7-4 in Draw 2.
Koe started his day with an 8-4 win over Olympic champion Brad Jacobs (1-1) of Sault Ste. Marie, Ont. Jacobs won his evening draw, 8-4 over Cotter.
On the women's side, Ottawa's Rachel Homan and Heather Nedohin of Edmonton are both 2-0 through the first day of the tournament.
Nedohin beat Val Sweeting of Edmonton 12-6 in the morning draw, then scored triples in the third and ninth ends to knock off Olympic champion Jennifer Jones (1-1) of Winnipeg 9-5 in the afternoon. Jones beat Alison Flaxey (0-2) of Listowel, Ont., 9-8 in their evening matchup.
Homan, the two-time defending Scotties Tournament of Hearts champion, beat Flaxey 11-3 for her second win of the day. Homan beat Sherry Middaugh (0-2) of Midland, Ont., 7-4 in the morning.
Sweeting (1-1) ended her day with an 8-2 win over Sherry Anderson (1-1). Anderson topped Middaugh 8-3 in Draw 2.
The Canada Cup format calls for the men's and women's teams to play a three-day round robin, with the first-place teams advancing to their respective finals on Sunday, while the second- and third-place teams meet in semifinals on Saturday, after any required tiebreakers, to determine the other finalists.