This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
You can store multiple objects in a table cell indirectly - through an array or collection. That won't render the way you want though.
For showing two ImageIcons you can use a workaround though - use a custom Icon implementation, and make getColumnClass return Icon.class instead of ImageIcon.class. This custom implementation is not hard (pseudo code):