The updateFTIndex() method is a method of the database object. To update the index on another database you first have to create a database object for the other database:
var dbOther:NotesDatabase = session.getDatabase( "", "pathToOtherDb.nsf"); //the first parameter indicates the server: this assumes that the database is on the same server
if (dbOther.isOpen() ) {
dbOther.updateFTIndex(false);
}
I don't know what access level you need to be able to perform an updateFTIndex call, but you could use the sessionAsSigner object:
var dbOther:NotesDatabase = sessionAsSigner.getDatabase( "", "pathToOtherDb.nsf");
Mark