According to the rental information on Tibia.com it doesn't give specifics as to which one would receive the letter and which one wouldn't. As others stated, it probably is random. Maybe they go based off of alphabetical order of the city or house name. Maybe they go off which house you owned first or last. As a previous comment mentioned they do post the exact second your rents due.
For example this is mine:
The house has been rented by Shawtay. She has paid the rent until Dec 27 2019, 10:03:45 CET.
The house has been rented by Shawtay. She has paid the rent until Dec 26 2019, 09:21:28 CET.
a) Rent
In addition to any one-time payment that is necessary to get a house, characters must pay a monthly rent for their houses. Rents can vary greatly depending on the size, location and furnishing of a house. Since landlords are suspicious people the rent must always be paid in advance. The money is automatically debited from the character's bank account. If it does not contain enough money when the rent is due, the owner of the house will receive a warning letter in the inbox for one week (starting at the day on which the rent is due). If the rent is not paid during this week, the character will be evicted from the house and all belongings in the house will be moved into the inbox of the character. Note, however, that large pieces of furniture that do not fit in a backpack will stay in the house. Furthermore, all characters on the player's account are blocked from renting another house for 30 days. The house concerned will be ready to be auctioned again.
https://www.tibia.com/gameguides/?subtopic=manual§ion=houses