Grouping radio buttons

I want to add 2 radio buttons to a group box which already contains 4 radio buttons.

Adding the buttons is straightforward, however, when I drag and drop them into the group box, although they work Ok together, they seem to work independently from the existing 4 buttons.

How can I get the new buttons to work with the existing buttons so that only 1 selection can be made from the 6 buttons.

make proper the 'tab order' of the controls in the dialog and enshure that no one of the radio buttons have a 'Group' flag set