I cam across this and found such a nice article.
Here is the situation
In support, we receive lots of calls with the question of
“I want to share free/busy information between two Microsoft Exchange Organizations. How can I accomplish this task?”
“I want to share GALs and Free/Busy information between two Microsoft Exchange Organizations.
SHARING GALs BETWEEN TWO MICROSOFT EXCHANGE ORGANIZATIONS
In order to share free/busy information, the very first thing you need to do is setup a GalSync Solution. The GalSync solution is a solution that is delivered, and can be configured inside of one of our Microsoft Identity and Access Management Products.