I haven't been to a Blue Jays-Expos so I can't tell for sure, but my guess would be that only the Canadian anthem is played. When two Canadian hockey teams play togheter, only the Canadian anthem is played, so I think the same rule apply for baseball (or any other sport).

>This reminds me of a question I've had for some time (and that's now moot). When the Expos and Blue Jays play, do they play only the Canadian national anthem before the game?