From the API Javadoc
public void setRowSelectionInterval(int index0,
int index1)
Selects the rows from index0 to index1, inclusive.
This sets the
current selection range
However, there is another method
addRowSelectionInterval
public void addRowSelectionInterval(int index0,
int index1)
Adds the rows from index0 to index1, inclusive, to the current selection.
This adds to the existing selection range.
Try using the second method