Question: Is multibinding not supported in Metro UI? 
Or has it just not been added yet ?

```xml
<TextBlock>
  <TextBlock.Text>
    <MultiBinding StringFormat="{}{0},{1}">
      <Binding Path="FirstName" />
      <Binding Path="LastName" />
    </MultiBinding>                
  </TextBlock.Text>                
</TextBlock>

```

Answer: 
I found a way of passing more then one parameter to OnExecute through using `Converter/ConverterParameters`.  

Here you can find more details:  

<http://mariuszgorzoch.wordpress.com/2014/07/21/metro-ui-multibinding/> 

In general I'm taking original `CommandParameter` and marge them with `ConverterParameters` in one array. The only limitation of that approach is that you can pass only one paramter `by-ref` and all other ones need to be in form of `string`.
You can download NuGet package for [WindowsStore](https://www.nuget.org/packages/WinRT-Multibinding-Windows/) projects. It supports most of WPF's MultiBinding features. You can find documentation on how to use it [here](https://github.com/Verbon/WinRTMultiBinding).