The conference hotel (Le Centre Sheraton Montreal) confirmed that they can split the bill any way you need: 50/50 or just split for the days that you and your roommate are sharing. You can tell the Front Desk upon checking in how the bill is to be split.
If you have a reservation, you can add your roommate's name to the reservation by calling the hotel. If need be, you can also give your roommate your confirmation number, so they can check in without you present.
As a reminder, if you are staying more nights than your roommate, you will be responsible for these on your own. If you have any questions, feel free to call the hotel front desk at (514) 878-2000. We will match you up as quickly as possible, and you can always cancel your reservation if your roommate has a reservation also.