I don’t think you can guarantee it but I’m pretty sure they would try to do their best to keep you in the same room for both reservations as long as you flag it up with them as soon as you check in. In fact if you can get hold of an e-mail address for the front desk of your hotel (they are out there if you search the web) give them a heads up about a week before.
However having said all that, I would be surprised if this saves you any money - it might even end up costing you more. Once you get to the 3rd or 4th day of your stay, the cost of adding on an extra day is almost totally down to the hotel cost; the ticket cost is virtually nothing in comparison. And a single day without tickets from a third party site could well be more than the package cost.