ADFS between 2 Domain Controller and need to share calender between 2 Exchange Servers

Dear All,

    My customer have 2 company, each company have own domain Controller Server (Domain name A.com and B.com). Each company have also own Exchange Server.

     - Domain A.com is Windows 2008R2 and Exchange 2007,

     - Domain B.com is Windows 2008R2 and Exchange 2010

     - Both company have 1GB link connect together.  

    My question, my 2 customer need to share calendar between Exchange 2007 and Exchange 2010 but they don't need to do 2 ways Trust Relationship between 2 Domain because they said can't acceptable on security.

     Could you please advise, I can use ADFS to federation between 2 DCs? If use ADFS, 2 Exchange Servers on different forest can share calendar?

      Or anyone have another way to answer this scenario? Could you please  advise

      Thank you very much.