Cannot add a vCenter 5.1 to an existing vSphere Web Client

Hello, we have currently 2 vCenter Servers in our network, each one with its own SSO and vSphere Web Client. I know that it is possible to have all the vCenters in the same Web Client but I have tried several methods without any luck.

Note that both vCenters are version 5.1

Any ideas anyone???

Thanks in advance

David