..figured it out :) I add this to the ExpandBar's parent Composite:
addControlListener( new ControlAdapter()
{
public void controlResized( ControlEvent e )
{
// get total (resized) size of control
int h = getSize().y;
// less the first components size
h -= component1.getSize().y;
// less the second components size
h -= component2.getSize().y;
// update the second expand items height (less 80 to account for
margins...)
expandItem2.setHeight( h - 80 );
}
} );