First you do not have to prove this unless they ask.
Each IRS auditor may ask different questions, but some of the things that they may look at are:
Who is paying the utilities, the upkeep, on the 2 family home.
May look at bank accounts to see money is being deposited that is not reflected on the return (ie, rental income).
There is nothing you need to send in on the return to prove this.
If I am not answering your question, please post a comment and provide more detail so I can help.