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>