First, your income is irrelevant if there is not a shared placement schedule. If you have the children 80% of the time, that is not a shared placement schedule. Second, even if there was, any income from the duplex would be only as stated on your tax returns. So, that would be income less expenses. You would be able to deduct any expenses from any income such as mortgage, real estate taxes, repairs/improvements, etc. Usually this is fairly nominal, if anything at all.
To be sure, though, you should consult with an experienced family law attorney.