linked universes are universes that share common components
such as parameters ,classes,objects and joins.among linked
universes one universe is said to be kernel or master while
the others are the derived universes.

if the data provided is from two different data providers
then we can link those two universes.such type of universes
are called as linked universes.

in real time senario we will work with linked universe.here 
same data source and diff data source is not a issue.we 
need to fech max 15 to 17 tables in single universe.more 
than that u will create another universe and link it.

file-->parametes -links

Concept of linked universes is based on inheritance. You 
can specify your common classed and objects in Parent 
universe (Kernal) and then link this universe in child 
universe (Derived Universe) to make use of these classes 
and objects. In Derived universe one can add their own 
tables,classes and objects.

However linking universe does not link following...

->Parameters
->contexts
->Aggregation