MSBuild AppXBundle for x86 and x64 - msbuild

Question

I want to build a *.appxbundle for a Windows 8.1 JavaScript app which contains the x86 AND x64 appx.
This is working fine on Visual Studio 2013. I right click on the project and choose Store->Create app packages and then pick both architectures.
However I can't get it working on MSBuild. My msbuild statement looks like this:
msbuild MyProj.Win81.jsproj /property:Configuration=Release;Platform=x64;AppxBundle=Always /target:Clean,Rebuild,Publish
My MyProj.Win81.jsproj looks like this (shortened a bit):
<?xml version="1.0" encoding="utf-8"?>
<Project DefaultTargets="Build" ToolsVersion="12.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<ProjectConfiguration Include="Debug|AnyCPU">
<Configuration>Debug</Configuration>
<Platform>AnyCPU</Platform>
</ProjectConfiguration>
<ProjectConfiguration Include="Debug|ARM">
<Configuration>Debug</Configuration>
<Platform>ARM</Platform>
</ProjectConfiguration>
<ProjectConfiguration Include="Debug|x64">
<Configuration>Debug</Configuration>
<Platform>x64</Platform>
</ProjectConfiguration>
<ProjectConfiguration Include="Debug|x86">
<Configuration>Debug</Configuration>
<Platform>x86</Platform>
</ProjectConfiguration>
<ProjectConfiguration Include="Release|AnyCPU">
<Configuration>Release</Configuration>
<Platform>AnyCPU</Platform>
</ProjectConfiguration>
<ProjectConfiguration Include="Release|ARM">
<Configuration>Release</Configuration>
<Platform>ARM</Platform>
</ProjectConfiguration>
<ProjectConfiguration Include="Release|x64">
<Configuration>Release</Configuration>
<Platform>x64</Platform>
</ProjectConfiguration>
<ProjectConfiguration Include="Release|x86">
<Configuration>Release</Configuration>
<Platform>x86</Platform>
</ProjectConfiguration>
</ItemGroup>
<Import Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" />
<PropertyGroup Condition="'$(VisualStudioVersion)' == '' or '$(VisualStudioVersion)' < '12.0'">
<VisualStudioVersion>12.0</VisualStudioVersion>
</PropertyGroup>
<Import Project="$(MSBuildExtensionsPath32)\Microsoft\VisualStudio\v$(VisualStudioVersion)\$(WMSJSProjectDirectory)\Microsoft.VisualStudio.$(WMSJSProject).Default.props" />
<Import Project="$(MSBuildExtensionsPath32)\Microsoft\VisualStudio\v$(VisualStudioVersion)\$(WMSJSProjectDirectory)\Microsoft.VisualStudio.$(WMSJSProject).props" />
<PropertyGroup>
<TargetPlatformIdentifier>Windows</TargetPlatformIdentifier>
<TargetPlatformVersion>8.1</TargetPlatformVersion>
<RequiredPlatformVersion>8.1</RequiredPlatformVersion>
<MinimumVisualStudioVersion>12.0</MinimumVisualStudioVersion>
<DefaultLanguage>en-US</DefaultLanguage>
<AppxAutoIncrementPackageRevision>False</AppxAutoIncrementPackageRevision>
<AppxBundle>Always</AppxBundle>
<AppxBundlePlatforms>x86|x64</AppxBundlePlatforms>
</PropertyGroup>
<ItemGroup>
<AppxManifest Include="package.win81.store.appxmanifest">
<SubType>Designer</SubType>
</AppxManifest>
</ItemGroup>
<Import Project="$(MSBuildExtensionsPath)\Microsoft\VisualStudio\v$(VisualStudioVersion)\$(WMSJSProjectDirectory)\Microsoft.VisualStudio.$(WMSJSProject).targets" />
</Project>
MSBuild will always generate a appxbundle which only includes the x64 architecture. This might be because I explicitly set Platform=x64, although AppxBundlePlatforms defines both. If I leave out the Platform parameter I run into a error:
error APPX3104: You cannot create an app bundle when building for platform 'AnyCPU' which is not included in the list of platforms selected for producting app bundle. Set platform to a one of following values: x86;x64.
What am I missing?
// edit: here a screenshot of VS2013 appxbundle creation and updated project configuration of jsproj file