rerendering commandbutton both top and bottom one

Visualforce Page

<apex:page extensions="MyController" standardController="Opportunity">
    <apex:form>
        <apex:pageBlock>
            <apex:pageBlockButtons location="top">
         <apex:commandButton value="Top" disabled="{!disableNextButton}" id="next_button1"/>
            </apex:pageBlockButtons>
          <apex:pageBlockButtons location="bottom">
         <apex:commandButton value="Bottom" disabled="{!disableNextButton}" id="next_button2"/>
            </apex:pageBlockButtons>
            <apex:pageBlockSection>
         <apex:inputField value="{!Opportunity.amount}">
           <apex:actionSupport event="onblur" 
                                 action="{!selectAssetAction}"
                                  rerender="next_button1, next_button2">
           </apex:actionSupport>
         </apex:inputField>
            </apex:pageBlockSection>
        </apex:pageBlock>
    </apex:form>
</apex:page>


Apex Class
public class MyController {
    Boolean disabled = false;
    public MyController(ApexPages.StandardController controller) {}
    public void selectAssetAction(){
        this.disabled = true;
    }
    public Boolean getDisableNextButton() {
        return disabled;
    }
}