It depends on what the agreement says. If you are paying rent under the rent-to-own agreement for the whole duplex and have the right to use, occupy, etc. the whole duplex, you can rent (or more propertly, sublease or subrent) out the other unit. If the rent-to-own agreement only gives you possession of your unit until you make the last payment and title transfers to you, then you can't rent the other unit out--you will have no rights to it in this scenario until you get title over the duplex. You have to review the proposed agreement to see what rights it gives you over the vacant unit in the meantime.