My company(CompanyA) just leased an office suite in building with another company(CompanyB). CompanyB owns the building and current has Exchang2010 and uses Room Mailboxes to schedule conference & training rooms, of which we have can book. I'd like an easy way to be able schedule their rooms in which it will show up in both our Exchange 2010 environments. CompanyB is willing to work with us on setting this up, but I really have no idea on how to start. Any suggestions?

You have posted in an Exchange 2013 forum.  The Exchange previous versions forums are here:  http://social.technet.microsoft.com/Forums/en-US/category/exchangeserverlegacy

You can federate free-busy between the two organizations.  You'll need to add a contact in your organization that has the e-mail address of the room mailbox in the other organization.

http://technet.microsoft.com/en-us/library/hh310374(v=exchg.141).aspx