Radio Buttons have an Initialize constructor that will allow you to define whether it is checked by default, whether it is "tri-state", and what its group name is. A group name is used to group more than one radio button together, so that if you check one, it will uncheck the previously checked one.
RadioButton rdbi1 = new RadioButton();
rdbi1.Initialize(ContainerType.HorizontalPanel, false, false, "metalsGroup");
rdbi1.BeginComposite<RadioButton>()
.AddText<RadioButton>(0, 0, "Iron")
.EndComposite<RadioButton, RadioButtonArgs>(null);
RadioButton rdbi2 = new RadioButton();
rdbi2.Initialize(ContainerType.HorizontalPanel, true, false, "metalsGroup");
rdbi2.BeginComposite<RadioButton>()
.AddText<RadioButton>(0, 0, "Steel")
.EndComposite<RadioButton, RadioButtonArgs>(null);
//Add the radio buttons to a Stack Panel
stkPanel1.BeginSettings<StackPanel>()
.Set<CheckBox, StackPanel>(0, 1, "Margin", new Thickness(5))
.Set<RadioButton, StackPanel>(0, 2, "Margin", new Thickness(5, 3, 5, 1))
.Set<RadioButton, StackPanel>(0, 3, "Margin", new Thickness(5, 0, 5, 3))
.EndSettings<StackPanel>();
stkPanel1.BeginComposite()
.AddExisting<RadioButton, StackPanel>(0, 2, rdbi1)
.EndComposite<StackPanel, StackPanelArgs>(null);
stkPanel1.BeginComposite()
.AddExisting<RadioButton, StackPanel>(0, 3, rdbi2)
.EndComposite<StackPanel, StackPanelArgs>(null);