Greenstone can search across several different collections as though they were
one. This is done by creating a "super-collection" that comprises the
individual collections. Under "Design Sections", click "Cross-Collection Search".
Simple Instructions:
The Cross-Collection Search view shows a checklist of available collections. The
current collection is ticked and cannot be deselected. To add another
collection to be searched in parallel, click it in the list (click again to
remove it). If only one collection is selected, there is no cross-collection
searching.
For further details, see Chapter 1 of the Greenstone Developer's Guide.