I make consecutive reservations at the same hotel pretty commonly, and this is never a problem. At check in the front desk will always confirm "You are staying ## nights." At that point I tell them: actually I have another reservation. The front desk looks it up and blocks my room for the future reservation.

The only time this can become an issue is if the hotel upgrades one reservation, but cannot upgrade the other. Whether you prefer to stay in the same room or take the upgrade and have to change rooms is a matter of personal preference.