This week's book giveaway is in the Mac OS forum. We're giving away four copies of a choice of "Take Control of Upgrading to Yosemite" or "Take Control of Automating Your Mac" and have Joe Kissell 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):