Installing Inventory Services 5.1

Building out a multi-site, multi-vcenter 5.1 farm.  I have a question about the Inventory Services 5.1 install.

Here is what we are doing:

Two Physical Sites

Each physical site will have a single SSO and Web Server

Each physical site will have two vCenter Servers connecting to its physical SSO

Now does Inventory Service need to be installed with each vCenter or can I install a single Inventory Service Server at each physical site like SSO and Web?

I have not been able to find much on the internet about this.

Thanks,

Boston Tech Guy