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.
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).