The following procedure will help you deploy a new PSC appliance and join it to an existing SSO domain for high availability or multi-site configurations:

1. Follow steps 1 through 11 from the section Part 1 – Deploying a PSC for a new SSO domain to deploy a new PSC.

2. Once deployed, on the Install - Stage 2: Set Up Platform Services Controller Appliance screen, click Next to continue.
3. On the Appliance configuration screen, configure Time synchronization and click Next to continue.
4. On the SSO configuration screen, choose the option to Join an existing SSO domain and supply the FQDN or IP address of the existing PSC appliance, the SSO domain name you intend to join, and the SSO password. Click Next to continue:

5. On the SSO Site Name screen, you can either choose to join an existing SSO site or create a new SSO site:
   • If you intend to join an existing site, then select Join an existing site and select the SSO site name from the drop-down box:

1. • If you intend to create a new site for a multi-site setup like SRM, then select Create a new site and supply an SSO site name:

6. With the desired option selected (in this case, I have chosen to create a new site Site-DR), click Next to continue. 
7. Choose whether or not to join VMware CEIP and click Next to continue.
8. On the Ready to complete screen, review the setting and click Finish.
9. The installer will warn you about the fact that you will not be able to pause or stop the installation once started. Click OK to continue.
10. Once the installation completes successfully, you will be presented with a screen indicating this. Click Close to exit the wizard.

The previous procedure (part 1 and part 2) will result in an environment with the PSCs psc01 and psc02 joined to the same SSO domain, vsphere.local, in a multi-site mode.