
First of all, keep in mind that each municipality and building type has there own rules. For instance, a 2-family residence may have different regulations than a 3-family, even in the same city. Or a 2-family in one city may have different rules than a 2-family in another city. So we are going to base this on generalities.
First of all, the short and simple answer is that the utilities should be separated, but that is not always the case. For the most part, the water is on one meter, and the tenants either split the bill, or the landlord pays the bill and rolls the cost into the rents. The landlord typically covers it though, otherwise it can create tension between tenants if one uses more water than the others.
As for gas, most of the time that will be completely separate and each tenant will have their own meter and pay their own bill based on their usage directly to the gas provider.
Electric is usually separated too, but not always completely. Most buildings have common areas and/or exterior lighting that either one of the tenants will have to pay, or the landlord will put in an electric panel called "the house panel" to cover basement lights, exterior lights, etc.
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