Generally - as long as your name is XXXXX XXXXX the title - as an owner or co-owner - you may deduct taxes and mortgage interest paid by you.
Your brother may deduct portion of taxes and mortgage interest paid by him.
There should not be double dipping - you may not deduct the same amount twice.
As long as you agree with your brother who will deduct and which amount - there should not be any problem - no any additional document needed to provide with your return.
If the IRS would question your return - you should be able to provide the proof that you own the property and you paid the portion of taxes and mortgage interest you reported as deduction.