Put all the radio buttons in the same field. If you need discrete groups of radio buttons on your layout, you can put the same field on your layout twice and format each copy with a different value list to get your two sets of radio buttons.
That's the simplest approach.
Otherwise, you can use an OnObjectModify script trigger to perform a script to clear the other field when a value in the current field is selected:
A single script step:
Set Field [YourTable::otherField ; ""]
Does the job. (And you'd need two triggers and two scripts to do this...
I went with the scripts but this works too and it's a heck of a lot easier.
You can Shift-click a radio button to clear it, but in some cases this may not clear the field. For example, a field called Color is formatted on Layout #1 to display two radio buttons, Black and White. The same field, Color, is formatted on Layout #2 as a text entry field that accepts any value. If a user has entered Red in the field on Layout #2, the information is now stored in the field, but doesn't display on Layout #1. To clear the field from Layout #1, select the field, then choose Edit menu > Clear
"Work smarter not harder"