<Style TargetType={x:Type local:MyControl}>
  <Style.Triggers>
    <Trigger Property="MyProperty" Value="MyValue1">
      <Setter Property="ControlTemplate">
        <Setter.Value>
          <ControlTemplate TargetType={x:Type local:MyControl}>
            <!-- first template -->
          </ControlTemplate
        </Setter.Value>
      </Setter>
    </Trigger>
    <Trigger Property="MyProperty" Value="MyValue2">
      <Setter Property="ControlTemplate">
        <Setter.Value>
          <ControlTemplate TargetType={x:Type local:MyControl}>
            <!-- second template -->
          </ControlTemplate
        </Setter.Value>
      </Setter>
    </Trigger>
  </Style.Triggers