It's on tape. And there are a lot of witnesses on all sides.
It is ingenious reasoning, but in the reality of politics I'm not sure it would happen like that, that is, that if the candidate in Hull—Aylmer supports a certain camp in the referendum and the candidate in the next riding for the same party supports the completely opposite option, I don't think one would be mounting a big campaign for the "yes" and the other for the "no". I think what would happen is that the candidate in Hull—Aylmer would get instructions from high up in the party so that their colleague's opponents in the next riding couldn't use the candidate's position to bolster their own. You see kind of what I mean.
What would probably happen is that in both cases the candidates would be urged not to say too much because they don't agree on the subject, and if either of them stated their opinion publicly the effect would be to embarrass both of them. I think that is what would happen.
Politics is your department, not mine. I think that in the example you cite, the parties would make sure their candidates did not say too much in expressing their opinions on the referendum. What happened in British Columbia and Ontario, where the candidates and members stayed silent on the referendum, illustrates that this is probably not a hypothetical scenario at all.