As we may be headed toward just such a scenario in the case of C-391, it is perhaps worth reviewing the precedent and procedure for tie votes in the House. So here goes.
Should a vote in the House result in a tie, it is the Speaker who holds what is known as the casting vote. The conventions covering this are well explained in House of Commons Procedure and Practice.
In theory, the Speaker has the same freedom as any other Member to vote in accordance with his or her conscience; however, the exercise of this responsibility could involve the Speaker in partisan debate, which would adversely affect the confidence of the House in his or her impartiality. Therefore, certain conventions have developed as a guide to Speakers (and Chairs in a Committee of the Whole) in the infrequent exercise of the casting vote. Concisely put, the Speaker normally votes to maintain the status quo. This entails voting in the following fashion:
-whenever possible, leaving the matter open for future consideration and allowing for further discussion by the House;
-whenever no further discussion is possible, preserving the possibility that the matter might somehow be brought back in the future and be decided by a majority of the House; and
-leaving a bill in its existing form rather than causing it to be amended.
Peter Milliken has had to make five such casting votes. On Oct. 8, 2009, he voted against a motion, allowing debate on a bill to continue. On April 30, 2009, May 4, 2005 and May 19, 2005 he voted in favour of bills, allowing them to proceed to committee—the latter being an unprecedented casting vote on a matter of confidence. On Sept. 16, 2003, he voted against an amendment to a motion (as further explained here).
If I were to hazard a guess, based on a reading of the above, as to what Mr. Milliken might do in this case—first on the motion to halt C-391 or, if that fails, on C-391 itself—my guess would probably be the same as this one offered by Kady O’Malley. But since neither of us are constitutional authorities, I’ll be dropping a note to a more authoritative source for further judgment.