@Tyler Brown You could bill the tenants for their usage specific to a known factor (square footage, occupants, etc). I've previously listed in another post some of the methods that utility management companies use to determine how much of the bill goes where. I've copied it below:
Common equations to determine utility billing:
Ratio based on Square footage: = Bill total * (Unit Square Footage / Total Square footage)
Example: Unit 1 has 1000 Sqft, Unit 2 has 500 sqft. Water bill for the month is $100.
Unit 1: $100 * (1000 sqft /1500 sqft) = $66.67
Unit 2: $100 * (500 sqft/1500 sqft) = $33.33
Ratio based on Number of occupants: = Bill Total * (Number of Occupants in Unit / Total Number of Occupants)
Example: Unit 1 has 2 tenants, Unit 2 has 3 tenants. Water bill for the month is $100.
Unit 1: $100 * (2 tenants / 5 tenants) = $40.00
Unit 2: $100 * (3 tenants / 5 tenants) = $60.00
Ratio based on half square footage, half occupants (50/50 method): = (Utility bill * 1/2)*( Unit Tenants / Total Tenants) + (Utility Bill * 1/2)*(Unit Sqft / Total Sqft)
Example: Unit 1 has 2 tenants, 1000 sqft. Unit 2 has 3 tenants, 500 sqft. Water bill for the month is $100.
Unit 1: ($100 * 1/2) * (2 tenants / 5 tenants) + ($100 * 1/2) * (1000 sqft / 1500 sqft) = $63.33
Unit 2: ($100 * 1/2) * (3 tenants / 5 tenants) + ($100 * 1/2) * (500 sqft / 1500 sqft) = $36.67
This third method is much more complex to do, but a simple Excel spreadsheet can help alleviate the headache of doing this monthly.
There are other ones you can do, like bathroom total ratios, dishwasher vs no dishwasher / washing machine vs no washing machine if the units differ.
In the end, you recoup the full cost of the utility bills and bill it as rent + Utility usage (variable) (Check with local landlord laws to determine if you are able to do this).
A majority of the time with a single family residence the utility company won't install a second meter to service your property. But, because you have a duplex, it's not unheard-of for utility companies to have a second meter to be installed at the property. It costs an arm, and half a leg, but at least you can remove the liability from your shoulders and make the tenants responsible for their usage.
Some other options that you could have is to install a water/gas/electric sub-meter system. These could be quite pricey, and are more common for larger multi-family units with one meter servicing an entire building. With a sub-meter ( a meter you have installed on the property side of the provider's meter) you can track usage of each unit, and know EXACTLY how to bill a property.
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EDIT: To answer your question, in my opinion, try to get it so the tenants can place services in their name, and make it so they are responsible for their usage. If the utility providers, or even General Contractors have issue with this, most states allow for up to a certain amount to be charged as a flat rate for utility usage alongside their rent. Check with an attorney proficient with landlord-tenant laws in your state / city and see if this is an option in your area.