Combo box with dual value list.

Hi

I need to implement a combo box derived class which can support a pair of lists (values) to be shown
when it is clicked  upon.  Which of the the list from a pair is shown is to be controlled by a flag.  

For e.g. Say the first list contains entries like USA, UK, Russia, Germany, France.
(When you expand combo box you will see each of the five countries as separate possible value.)
Second list will contain values like USA, UK, Russia, Germany, France, India, Pakistan, China.
First list will always be subset of the second list.

Which of the two value shows up is simply controlled by boolean flag.
If flag set to True, it  will show the first list in the pop up box else it will show the second list set.

I want to avoid having to maintain two pairs of list and changing the combo box contents everytime.
Among other things it might screw up other part of code that depend upon the index/position of value
on the list.