Not that it helps, as I am unsure of the laws in NJ but the answerer above has it incorrect. Laws can (and sometimes do) dictate what utilities can be split. In Philadelphia it is illegal to have a tenant pay any portion of the natural gas bill if the unit is not on a separate meter. In an instance where there is no separate meter the landlord can charge a higher rent and include the cost of the utility in the rent price but he/she cannot charge a percentage of the bill based on the square footage/number of people living in/etc the unit.
Not that it answers your question, I just wanted to point out that occasionally the law does step in for these things as another answerer stated it does not.
Well if it has a separate entrance, kitchen and only one meter.
It was probably built as a mother-daughter house. You could try to have the utilities install separate meters. Otherwise you could divide the bills proportionately by square footage and approximate hours each one is home. And have it written into the lease. If it's not written in I don't think the landlord will be soley responsibale. It's kind of like saying police can't chase you across state lines, but they can