C / C++ / MFC

// Assuming you have assigned member variables for the 2 comboboxes

// Create a handler for the "Add" button.
void CYourDialog::OnAddButton()
{
   CString cText;
   int     nIndex;

   nIndex = m_ComboboxA.GetCurSel();     // store the selected index
   if(nIndex == CB_ERR)                  // exit if there is no selection
      return;
      
   m_ComboboxA.GetLBText(nIndex, cText); // store the selected text in cText
   m_ComboboxB.AddString(cText);         // now insert that text into combobox B
}


// Create a handler for the "Remove" button.
void CYourDialog::OnRemoveButton()
{
   int     nIndex;

   nIndex = m_ComboboxB.GetCurSel();     // store the selected index
   if(nIndex == CB_ERR)                  // exit if there is no selection
      return;

   m_ComboboxB.DeleteString(nIndex);     // remove the selected string
}