How do I include a legal clause on split utilities?

[Casity Kao]

We have a single family home with two levels and will rent out to two tenants. Gas electricity and water are all one meter. Utilities will be under our name and we will bill tenants. Lower level is 950sqft and upper level is 575sqft. The split will be 60% for lower and 40% for upper. How can we legally include a clause into the rental contract to bill both tenants? Any recommendations for legal terminology we should be using is greatly appreciated!