This below process is for Presentation Server 4.5

Open the first farm and:

Open the Access Management Console at Web Interface Server, click “Configuration Tool” -> “Web Interface” –> right click on the site you created and select “Manage Server Farm” -> Add second Farm

In Citrix Xenapp 5.0,

Open AMC and select the website you have created, right click and select Manage Server Farm–> Add second farm.