Give the ones you want to act as a group the same name.
What groups radio buttons is how you name them. All of the radio buttons in the same group have exactly the same name.
But each button in the group have a different "button value" (at the bottom of the Buttons & Forms panel. That's the information that's exported when collecting the form data.
From the Buttons and Forms panel menu, choose Sample Buttons and Forms. Look at the radio button groups and how they are named.
The trick to radio buttons is to remember "same name, unique values".
For example, if the question is "Do you love InDesign?" and the anwers are yes and no:
If the next question is do you love Illustrator:
THANK YOU EVERYONE> SOUNDS SIMPLE, BUT I WAS BEING AN IDIOT. THANK THANK THANK YOU ALL
Has anyone found a new solution to this problem, Adobe has screwed same name buttons in InDesign. When exporting it renames all adding 1, 2, 3 next to them. This means you can't get them to act as one, YES/NO buttons can now be both selected. Before the update you could limit it to either yes or no, instead of both. Editing in Acrobat is also blocked.