If I have a radio button defined as below everything works ok.  The field ratePlanId is updated with the selected value.

<apex:page controller="MyController">

   <apex:form>

      <apex:pageBlock>

           <apex:pageBlockSection id="ratePlanTable" title="Rate Plans">

                <apex:selectRadio id="ratePlanSelector" layout="pageDirection" value="{!ratePlanId}" required="true">

                     <apex:selectOptions value="{!ratePlans}"/>

                </apex:selectRadio>

            </apex:pageBlockSection>

       </apex:pageBlock>

    </apex:form>

</apex:page>

when I modify the page and add an additional radio button as below the ratePlanId setter is never called.  Oddly enough, from the debug log it looks the getter is called again instead of the setter.

             :  page block section with first radio button located here

             <apex:pageBlockSection id="ratePlanChargeTable" title="Rate Plan Charges">

                    <apex:selectRadio layout="pageDirection" value="{!ratePlanChargeId}" required="true">

                       <apex:selectOptions value="{!ratePlanCharges}"/>

                   </apex:selectRadio>

             </apex:pageBlockSection>

        </apex:pageBlock>

    </apex:form>

</apex:page>

Why does the presence of a second radio buttion cause the first radio button to no longer set a value when selected?