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: 
Unfortunately, MultiBinding doesn't exist in Metro XAML yet in the Beta and given that few new features are added after Beta, it most likely won't show up until the next version. The StringFormat you're using here is also not available yet on Binding.

In general, the maturity of the Metro XAML feature set is currently similar to Silverlight from a few versions ago, so compared to SL 5 it's missing some things and compared to WPF it's missing a lot, but on the plus side this stuff should get added in fairly quickly in future versions.
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`.