What form do i need for proof of ownership of a 2 unit rental property?

Who is asking for a proof of ownership? Is this related to an IRS audit of your return?

An EIN is not required for reporting rental property activity on your tax return unless you have payroll expense, which is fairly uncommon.

Your proof of ownership is the deed.  Why do you think you need to prove it ???  Nowhere on the Sch E is this asked. 

I am applying with EIDL for a grant. They are asking for an EIN number for them to approve. I thought about the tax papers and copies of the lease for proof, but wasn't sure if that would work.

You don't get an EIN for a passive rental reported on a Sch E ... you may have entered the rental income as earned income on the application.   Ask them to clarify what is being asked for since what you want doesn't exist.