How can I use a single Visual Studio solution to build both x86 and x64 at the same time?

We do something similar to build core assemblies for .NET Compact Framework.

Try this:

<Target Name="AfterBuild">
    <MSBuild Condition=" '$(Platform)' == 'x86' " Projects="$(MSBuildProjectFile)" Properties="Platform=x64;PlatFormTarget=x64" RunEachTargetSeparately="true" />
</Target>

Importing a project in such manner works for me in Visual Studio 2010:

TestProject64.vcxproj

<?xml version="1.0" encoding="utf-8"?>
<Project DefaultTargets="Build" ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <Import Project="TestProject.vcxproj" />
  <ItemGroup Label="ProjectConfigurations">
    <ProjectConfiguration Include="Release|x64">
      <Configuration>Release</Configuration>
      <Platform>x64</Platform>
    </ProjectConfiguration>
  </ItemGroup>
  <PropertyGroup Label="Globals">
    <ProjectGuid>{B7D61F1C-B413-4768-8BDB-31FD464AD053}</ProjectGuid>
  </PropertyGroup>
</Project>

TestProject64.vcxproj.filters

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <Import Project="TestProject.vcxproj.filters" />
</Project>

TestProject.vcxproj has two configurations defined inside: Release|x86 and Release|x64. As you can see, TestProject64.vcxproj has only the Release|x64 configuration. Defining of at least one configuration in TestProject64.vcxproj is necessary, otherwise Visual Studio will not be able to add TestProject64.vcxproj to a solution.

Now it's possible to include both TestProject.vcxproj and TestProject64.vcxproj to the same solution and build Release|x86 and Release|x64 at the same time.

I think the best way of doing this is to invoke MSBuild from the command line. It shouldn't need editing of MSBuild files. Just run

msbuild myproj.sln /p:Configuration="Debug|Win32"
msbuild myproj.sln /p:Configuration="Debug|x64"

I assume that if a developer is using Visual Studio then they'll only be generating the DLL files so they can debug with them, and that you have a separate build process if you're actually deploying the DLL files.