How do I use UpdatePanel Trigger with a control outside of panel

Question

I have an issue where I need to trigger 2 update panels to update when the user clicks a button which is OUTSIDE of both of these panels

<asp:UpdatePanel runat="server" ID="pnlDropDowns" UpdateMode="Conditional">
  <Triggers>
    <asp:AsyncPostBackTrigger ControlID="btnUseMeter" Event="Click"/>
  </Triggers>
<ContentTemplate>


<asp:UpdatePanel runat="server" ID="pnlChart" UpdateMode="Conditional">
  <Triggers>
    <asp:AsyncPostBackTrigger ControlID="btnUseMeter"  Event="Click"/>
  </Triggers>
<ContentTemplate>

<asp:Button ID="btnUseMeter" runat="server" Text="View Meter Data" Enabled="True"   OnClientClick="lowerFlag();" />                                             

I get the error

A control with ID 'btnUseMeter' could not be found for the trigger in UpdatePanel 'pnlDropdowns'.

Is there a way round this? Is it possible to use a hidden field? If so what would the event be? I could possibly set the value of a hidden field to say when I want the panels to update?

I have a considerable amount of HTML on this page so restructuring the page into a single panel is not an option, nor is moving the button to be inside either of the panels

I am using C# .NET 4.5

Cheers

Paul