go to universe parameters and select link tab and then
select the universe.

Universes can be linked using Universe Parameters. You can 
link universes with any of the following approaches:
1)Kernel approach
2)Master approach
3)Component approach

With the Kernel approach, one universe contains the core 
components. The derived universes that you create from this 
kernel universe contain these core components as well as 
their own specific components. Any changes you make to the 
kernel universe are automatically reflected in the core 
components of all the derived universes.

The Master approach is another way of organizing the common 
components of linked universes. The master universe holds 
all possible components. In the universes derived from the 
master, certain components are hidden depending on their 
relevance to the target users of the derived universe. The 
components visible in the derived universes are always a 
subset of the master universe. There are no new components 
added specific to the derived universe. Any changes you 
make to the master universe are automatically reflected in 
the core components of all the derived universes.

The Component approach is merging two or more universes 
into one universe.

we have a concept in BO linked universe there we can klink 
multiple universes.
file--.parameters-->links