Have a look at this example: http://assure.ly/j1KdNq. It works on the basis of the user holding the Shift button while clicking the radio buttons. Obviously the user will need an instruction on this.
Hope that helps,
A simple work-around that I have found helpful is to use two checkboxes instead of a pair of radio buttons. You can use the action builder to create actions so that when one is checked the other is set to null or unchecked. This way, they function like radio buttons but you retain the ability to deselect them. You can then change them to appear as sunken circles so that they do not look any different.
Here is how to build your actions:
Add two check boxes to your form. It is helpful to name them in the hierarchy for ease of use.
Right click on checkbox 1 and select Actions>Add Action. The Action Builder window opens.
Under CONDITION, set the condition to be "When CheckBox1 is checked" by choosing "is checked" from dropdown. If you opened the Action Builder window without right clicking on the CheckBox object, then you will also need to define which object you are editting
In Select a Result Dropdown, choose "Set the Value of a Field" and select CheckBox2 from hierarchy (a window opens to display the heirarchy to select an object from)
Change the result field dropdowns to be: Set the value of CheckBox2 "TO BE" "UNCHECKED"
Repeat these actions for the second CheckBox.
Afterwards, select each checkbox, and in the Object window, select the Field tab. From there, there is an Appearance dropdown. Use that to select the appearance of your CheckBox. I believe that Sunken Circle is the standard appearance for radio buttons
Radiobuttons are based on an exclusion group. one will be always selected as they are mutually exclusive. Sounds like the checkBox solution is the right one here for this use case, as you can select multiple, one, or none. You could also provide a third choice representing "no selection".
I Im currently building a form with multiple radio button groups with child radio button groups and have it working
a checkbox to say user wants that section ( if unchecked clears all radio buttons)
if a user clicks a child it must turn on the parent etc
so here is some code examples I used
on the radio button group under change
if ( pg1.suby.subrad.rawValue > 0 )
pg1.rad1.rawValue = 1
if ( pg1.rad1.rawValue > 1 )
basically if subrad has any value then rad 1 needs turned on
if the paren is set to anything but one the reset this sub group
if you need a full example of a parent with 3 choices each with a 3 child choices and master check box let me know i figured that short exerpt should sufice though