2 Family Up and Down, Son lives up , Mother lives Down

[Brad Fausett]

My question is can I ask them to pay water and sewer and divide it between them?   The current situation is they both live together upstairs, mother wants to move into the newly vacated down unit but we are not quite at the same rental evaluation.  I was going to offer them a better price but let them share and split the water bill for the units.  Is this legal?