I created an addendum in this situation and each unit was billed based on pro-rate of the square footage.
Utilities:
Tenant agrees to ratio utility billing where utilities remain in Landlords name and monthly bill pro-rated and billed to Tenant.
Tenant in lower level unit #A will be billed based off of 900 total square footage, of each bill.
Tenant in upper level unit #B will be billed based off of 2,537 total square footage, of each bill.
Utility charges will need to be paid in full by the following first of the month. Funds will be automatically applied to unpaid utilities prior to rent.
In the event unit #A is vacant, utility charges will be the responsibility of Tenant in unit #B with a 5% reduction of bill*, until unit #A is occupied and then billing resumes on a pro-rated portion, as described above.
*5% of utility bill to be paid by Landlord while unit #B is vacant. Once unit #A is occupied, and then billing resumes on a pro-rated portion, as described above.