Justin,
The rule of thumb for rentals is that expenses will take up 50% of your rent. Search for "50% rule" to read other posts on this topic. Even when you live in a home you own, you incur some of those expenses (utilities, taxes, insurance, maintenance) though not others (property management, vacancies, legal costs, advertising, make ready between tenants, etc.) We don't always track them very carefully, but they're there.
If by free, you mean want the rent on one half to cover your mortgage, that's a straightforward calculation. If you want the rent to cover the mortgage and all the expenses, then you need a much better deal. Figure that your expenses on the rented side will be 50% of the rent. Since you avoid some on your side, figure 35% of what the rent would be. If the two halves are the same, then 85% of the rent on the rented side goes to the total expenses. Then you need to keep the payment down to 15% of the rent. That may be a challenge.