In most circumstances, control descriptions do not have more than one symbol in each box.  However, if you need to place a control using two features, e.g. between two cliffs or hills, you may want to show two symbols in the same control description box.

1. First open a window to edit the control in question. 
2. Then select the relevant control description field. 
3. Finally, use the button "Two symbols" in the toolbar below the symbol palette to split the field into two parts. 

The two parts of the field can now be selected independently of each other and may contain different symbols.

Use the button "One symbol" to revert the field to holding one symbol.