In order to make it work, you have to set property GroupName of both radio buttons to the same value:
<asp:RadioButton id="rbMetric" runat="server" GroupName="measurementSystem"></asp:RadioButton>
<asp:RadioButton id="rbUS" runat="server" GroupName="measurementSystem"></asp:RadioButton>
Personally, I prefer to use a RadioButtonList:
<asp:RadioButtonList ID="rblMeasurementSystem" runat="server">
<asp:ListItem Text="Metric" Value="metric" />
<asp:ListItem Text="US" Value="us" />
</asp:RadioButtonList>