Problem getting gridbag to give me symetry

Hi all,

I can't seem to get the gridbag layout to stop adjusting the size of cells relative to each other.  I have two lists side by side, and I want them to split the display exactly 50/50 no matter what.  

When neither has anything in, they are the same size.  However, when one of them gets some list elements, the amount of x axis room allocated to it increases, making the display lopsided.  Is there a way to stop this happening?  I thought that was the point of the weights?  

Here is the code (note: This isn't the exact code - I've cut several of the UI components out to keep it simple but the principle is the same)

    buildConstraints(constraints, 0, 0, 1, 1, 50, 100);
    constraints.fill = GridBagConstraints.BOTH;
    JList list1 = new JList(nameList);
    list1 .setLayoutOrientation(JList.VERTICAL);
    list1 .setVisibleRowCount( -1);
    JScrollPane pane1= new JScrollPane(
      list1 , JScrollPane.VERTICAL_SCROLLBAR_ALWAYS, JScrollPane.HORIZONTAL_SCROLLBAR_NEVER);
    gridbag.setConstraints(pane1, constraints);
    mainPane.add(pane1);

    buildConstraints(constraints, 1, 0, 1, 1, 50, 0);
    constraints.fill = GridBagConstraints.BOTH;
    JList list2 = new JList();
    list2 .setLayoutOrientation(JList.VERTICAL);
    list2 .setVisibleRowCount( -1);
    JScrollPane pane2 = new JScrollPane(
      list2 , JScrollPane.VERTICAL_SCROLLBAR_ALWAYS, JScrollPane.HORIZONTAL_SCROLLBAR_NEVER);
    gridbag.setConstraints(pane2 , constraints);
    mainPane.add(pane2 );