It would be hard, because it interferes with the normal operation of the ID. It would be more natural to add a compound piece to the table, just as the new version of the Play-Test Applet allows you to duplicate a piece in the table, in case you need two types with the same image.
So a way to do it could be this: You select one piece in the table (to be used as front icon, say), and then there is a button 'Combine' above the table. When you press that, the next piece in the table you click will then not be selected, but combined with the already selected piece. And this combo will be inserted just below the piece you were clicking. You can then place that new piece wherever you want, in the normal way.
It would be hard, because it interferes with the normal operation of the ID. It would be more natural to add a compound piece to the table, just as the new version of the Play-Test Applet allows you to duplicate a piece in the table, in case you need two types with the same image.
So a way to do it could be this: You select one piece in the table (to be used as front icon, say), and then there is a button 'Combine' above the table. When you press that, the next piece in the table you click will then not be selected, but combined with the already selected piece. And this combo will be inserted just below the piece you were clicking. You can then place that new piece wherever you want, in the normal way.