You define multiple cubes only in these cases
1) Security becomes easier to handle when there are different cubes for different users
2) YOu can split the business dimensional space into two sub spaces of less dimensionality, with few dimensions overlapping between them
3) You have different sets of measures that are local in different cubes although the business dimensional space is similar. IN this case you can even have the same cube but different measure group.
Vijay
