Use
RadioButton.GroupName
or
RadioButtonList
control.
http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.radiobutton.groupname.aspx[
^]
http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.radiobuttonlist.aspx[
^]
[Edit: Forms Application]
The RadioButton control can display text, an Image, or both. When the user selects one option button (also known as a radio button) within a group, the others clear automatically. All RadioButton controls in a given container, such as a Form, constitute a group. To create multiple groups on one form, place each group in its own container, such as a GroupBox or Panel control.
Ref:
http://msdn.microsoft.com/en-us/library/system.windows.forms.radiobutton.aspx[
^]
So, if you want to select both the radiobutton then you have to put them on different containers like, panel or groupbox.