I found that this works:
Display the Hieracrhy. There you'll probably see that you have two radio button lists.
Select each of the radio button lists by holding the shift key when you click on them.
Then right-click (still within the Hierarchy area), which will display some menu options.
Select Merge Radio Button Groups.
You'll then see that the two buttons are wrapped together. You'll probably also see a little red warning symbol, which will tell you that you have two buttons with the same value. To correct that, open the Object pallet and select the Binding tab. The values of your two buttons should be displayed here. If you don't see them, click on the Specify Item Values box.
You will see that both items have a value of "1". Double-click on the second one and change the "1" to a "2". That should fix it.
There may be a simpler way to do this but I'm just starting out, too, and this is the way that works for me. Hope it helps!